Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations

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Conservation of Mass

The law of conservation of mass states that no matter is lost or gained during a chemical reaction.
The total mass of the reactants equals the total mass of the products.
This means that all chemical equations must be balanced.
In a closed system, no substances can enter or leave, so the total mass remains constant.
In an open system, a gaseous product can escape, causing the total mass to decrease.
If the mass of a reaction flask increases, it is likely that a gaseous reactant from the air has been used and all products are solids or liquids.

A sealed bag of vinegar and baking soda has a mass of 34g before and after the reaction.

Balanced Chemical Equations

A balanced equation has the same number of atoms of each element on both sides.
You balance equations by changing the coefficients (large numbers in front of formulas), never by changing subscripts.
A coefficient applies to the entire formula that follows it. For example, 2H₂O means 4 hydrogen atoms and 2 oxygen atoms.
If a group of atoms (like NO₃) remains unchanged, count it as a single entity.
Balance elements that appear on their own last.
The following non-metals are written as molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

A balanced chemical equation

Interpreting Chemical Formulas

If there is no subscript after an element, there is one atom of that element.
A subscript number applies only to the element immediately before it.
A subscript after brackets applies to all elements inside the brackets.
For example, Ca(OH)₂ contains 1 calcium, 2 oxygen, and 2 hydrogen atoms.
In Ca(NO₃)₂, the subscript 2 outside the brackets multiplies everything inside: 1 calcium, 2 nitrogen, and 6 oxygen atoms.

Relative Formula Mass (Mr)

The relative atomic mass (Ar) of an element is the larger number shown on the Periodic Table.
The relative formula mass (Mr) is the sum of the relative atomic masses of all atoms in the formula.
For example, Mr of H₂O = (2 × 1) + 16 = 18.
For Ca(OH)₂: Mr = 40 + (2 × 16) + (2 × 1) = 74.
In a balanced equation, the sum of the Mr values of the reactants equals the sum of the Mr values of the products.

The Periodic Table key showing where relative atomic mass (Ar) and atomic number (Z) are given for each element, using carbon as the example.

Percentage by Mass

The percentage by mass of an element in a compound is calculated using: (Ar × number of atoms of the element) ÷ Mr of the compound × 100.
For water, the percentage of hydrogen is (2 × 1) ÷ 18 × 100 = 11.1%.
The percentage of oxygen in water is (1 × 16) ÷ 18 × 100 = 88.9%.
The percentages of all elements in a compound must add up to 100%.
A common error is to put the numbers upside down, giving a percentage greater than 100%, which is impossible.

Mass Changes with Gases

Some reactions appear to involve a mass change because a reactant or product is a gas.
If a gaseous product escapes from an open system, the total mass decreases.
When a metal reacts with oxygen, the mass of the oxide produced is greater than the mass of the metal because oxygen from the air is added.
When a metal carbonate thermally decomposes, carbon dioxide is released, leaving the metal oxide as the only solid product.
For example, in the reaction 2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g), mass is lost as CO₂ escapes.
In a closed system, no gas can escape, so the mass remains constant.

Chemical Measurements and Uncertainty

Every measurement has some uncertainty.
Random errors cause results to be scattered around the true value, sometimes too high and sometimes too low.
Systematic errors always shift results in the same direction, e.g. a balance not zeroed.
For an analogue instrument, uncertainty is ± half the smallest division.
For a digital instrument, uncertainty is ± the smallest division.
For repeated measurements, uncertainty can be estimated as ± half the range of results.

Calculating Mean and Uncertainty

The mean is the sum of all measurements divided by the number of measurements.
The range is the difference between the highest and lowest values.
Uncertainty from repeated measurements is calculated as range ÷ 2.
For example, if repeat results are 2.9 g, 2.7 g, and 2.8 g, the range is 0.2 g, so uncertainty is ±0.1 g.
Always report a measurement with an appropriate number of significant figures, matching the precision of the instrument.

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1.What does the law of conservation of mass state?
Easy
- ANo matter is lost or gained during a chemical reaction.
- BMass is always lost when a gas is produced.
- CThe mass of the products is always greater than the mass of the reactants.
- DAtoms are created and destroyed during a chemical reaction.
2.In the formula Fe2O3, how many atoms are shown in total?
Easy
- A5
- B2
- C3
- D6
3.How many atoms are present in one formula unit of iron(II) nitrate, Fe(NO3)2?
Easy
- A9
- B7
- C5
- D11
4.In a chemical reaction, the total number of atoms stays the same.
Easy
True or false?
5.Which of the following statements about a balanced chemical equation are correct? (select all that apply)
Medium
- AThe number of atoms of each element is the same on both sides.
- BYou can balance an equation by changing subscripts in a formula.
- CThe total mass of reactants equals the total mass of products.
- DCoefficients (multipliers) in front of formulae can be changed to balance the equation.
- EAtoms are converted into energy during the reaction.
6.What is the relative formula mass (Mr) of calcium hydroxide, Ca(OH)2? (Ar: Ca=40, O=16, H=1)
Medium
- A74
- B57
- C58
- D80
7.A student heats magnesium in an open crucible. The mass of the contents increases. Why?
Medium
- AOxygen from the air reacts with the magnesium to form magnesium oxide.
- BMagnesium atoms are converted into energy.
- CSome magnesium evaporates and is lost.
- DThe balance is faulty and always reads high.
8.When calcium carbonate is heated in an open test tube, the mass decreases. Which statement best explains this?
Hard
- ACarbon dioxide gas is produced and escapes into the air.
- BCalcium atoms are destroyed during the reaction.
- COxygen from the air reacts with the calcium carbonate.
- DWater vapour is absorbed from the air.

更多題目

9.In the reaction: 2Mg(s) + O2(g) → 2MgO(s), the mass of the reaction flask increases when the reaction is carried out in an open system. Why?
Medium
- AOxygen gas from the air is a reactant and is added to the system.
- BMagnesium gas escapes from the flask.
- CThe reaction produces a gas that is lost.
- DThe magnesium atoms gain mass from the air.
10.What is the uncertainty of a digital balance that reads to 0.01 g?
Medium
- A±0.01 g
- B±0.005 g
- C±0.1 g
- D±0.02 g
11.A student records the following masses for a reaction: 2.9 g, 2.7 g, 2.8 g. Estimate the uncertainty in these readings.
Medium
- A±0.1 g
- B±0.2 g
- C±0.05 g
- D±0.3 g
12.Systematic errors always pull the result away from the accepted value in the same direction.
Easy
True or false?
13.Match each term with its correct definition.
Medium
- Conservation of mass
- Relative formula mass
- Uncertainty
- The sum of the relative atomic masses of the atoms in the formula.
- No matter is lost or gained during a chemical reaction.
- The confidence to which a measurement can be taken.
14.Place the steps for balancing a symbol equation in the correct order.
Medium
- Balance elements that appear on their own, last.
- Work across the equation from left to right, checking one element after another.
- Change coefficients (multipliers) in front of formulae, one by one.
- Check that the number of atoms of each element is equal on both sides.
15.A student uses a digital balance that reads to 0.001 g. What is the uncertainty of this balance?
Easy
- A± 0.001 g
- B± 0.0005 g
- C± 0.01 g
- D± 0.002 g
16.A thermometer has an analogue scale with markings every 0.5 °C. What is the uncertainty of a temperature reading taken with this thermometer?
Easy
- A± 0.5 °C
- B± 0.25 °C
- C± 1.0 °C
- D± 0.1 °C
17.In a rate of reaction experiment, a student records the following masses lost at 60 seconds: 2.9 g, 2.7 g, 2.8 g. Estimate the uncertainty in these readings.
Medium
- A± 0.05 g
- B± 0.1 g
- C± 0.2 g
- D± 0.5 g
18.Which of the following statements about random and systematic errors are correct? (select all that apply)
Medium
- ARandom errors pull a result away from the accepted value in either direction.
- BSystematic errors always pull the result away from the accepted value in the same direction.
- CRandom errors are the same as uncertainties.
- DSystematic errors can be reduced by taking more readings and calculating a mean.
- EForgetting to zero a balance is an example of a random error.
19.Which of the following changes would cause the mass of the reaction mixture to decrease when carried out in an open container? (select all that apply)
Hard
- AHeating calcium carbonate to produce calcium oxide and carbon dioxide.
- BAdding magnesium to dilute hydrochloric acid, producing hydrogen gas.
- CHeating magnesium in air to form magnesium oxide.
- DAdding copper carbonate to dilute hydrochloric acid, producing carbon dioxide.
- EBurning iron wool in oxygen to form iron oxide.
20.Which of the following are true about a balanced chemical equation? (select all that apply)
Medium
- AThe number of atoms of each element is the same on both sides.
- BThe sum of the relative formula masses of the reactants equals the sum of the relative formula masses of the products, taking coefficients into account.
- CYou can balance an equation by changing the subscript numbers in a chemical formula.
- DThe total mass of the reactants equals the total mass of the products.
- EThe number of molecules is always the same on both sides.
21.When a gas escapes from an open reaction vessel, the mass of the vessel and contents decreases.
Easy
True or false?
22.In a chemical reaction, the total number of atoms always stays the same.
Medium
True or false?
23.The relative formula mass of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
Medium
True or false?
24.Match each term with its correct description.
Medium
- Conservation of mass
- Relative formula mass
- Uncertainty
- Systematic error
- The sum of the relative atomic masses of the atoms in the formula
- No atoms are lost or made in a chemical reaction
- An estimate of the confidence in a measurement, often half the range of repeated results
- An error that always pulls results in the same direction
25.Match each observation to the correct explanation.
Medium
- Mass increases when magnesium is heated in air
- Mass decreases when calcium carbonate is heated in an open test tube
- Mass stays the same when lead nitrate and potassium chromate solutions react in a closed flask
- Mass decreases when copper carbonate reacts with acid in an open flask
- Oxygen from the air is added to the magnesium
- Carbon dioxide gas escapes to the atmosphere
- A precipitate forms and no gas escapes
- Carbon dioxide gas is produced and escapes
26.Place the steps for balancing the equation Al + CuO → Al2O3 + Cu in the correct order.
Medium
- Balance aluminium atoms: place a 2 in front of Al.
- Balance oxygen atoms: place a 3 in front of CuO.
- Balance copper atoms: place a 3 in front of Cu.
- Check that all atoms are balanced.
27.Place the steps for calculating the percentage by mass of an element in a compound in the correct order.
Medium
- Calculate the relative formula mass (Mr) of the compound.
- Multiply the relative atomic mass (Ar) of the element by the number of its atoms in the formula.
- Divide the total mass of the element by the Mr of the compound.
- Multiply by 100 to get a percentage.
28.A student uses a digital balance that reads to 0.01 g. What is the uncertainty of this balance?
Easy
- A±0.005 g
- B±0.01 g
- C±0.02 g
- D±0.1 g
29.A thermometer has an analogue scale with markings every 1 °C. What is the uncertainty of a temperature reading taken with this thermometer?
Easy
- A±0.1 °C
- B±0.5 °C
- C±1 °C
- D±2 °C
30.Which of the following is the relative formula mass (Mr) of copper oxide, CuO? (Ar: Cu = 63.5, O = 16)
Easy
- A47.5
- B79.5
- C95.5
- D143
31.What is the relative formula mass (Mr) of magnesium nitrate, Mg(NO3)2? (Ar: Mg = 24, N = 14, O = 16)
Medium
- A86
- B134
- C148
- D172
32.What is the percentage by mass of oxygen in water, H2O? (Ar: H = 1, O = 16)
Medium
- A11.1%
- B16.0%
- C88.9%
- D94.1%
33.A student heats a metal carbonate in an open crucible. The mass of the crucible and contents decreases. Which statement best explains this?
Medium
- AThe metal carbonate reacts with oxygen in the air.
- BCarbon dioxide gas is produced and escapes into the atmosphere.
- CThe metal carbonate melts and some liquid evaporates.
- DThe metal oxide produced has a lower relative formula mass than the carbonate, so mass is lost.
34.In the reaction 2Mg(s) + O2(g) → 2MgO(s), the mass of the reaction flask increases when the reaction is carried out in an open system. Which statement best explains this?
Medium
- AOxygen from the air reacts with magnesium, adding mass to the solid product.
- BMagnesium reacts with nitrogen in the air to form a nitride.
- CSome magnesium evaporates and then condenses back.
- DThe magnesium oxide produced has a lower mass than the magnesium because oxygen is lost.
35.A student mixes two solutions in a closed flask and a precipitate forms. The mass before and after the reaction is the same. Which statements explain this observation? (select all that apply)
Hard
- ANo atoms are lost or made during the reaction.
- BThe total mass of the reactants equals the total mass of the products.
- CThe reaction is a precipitation reaction.
- DThe flask is a closed system, so no substance can enter or leave.
- EA gas is produced and escapes from the flask.
36.A student investigates the reaction between copper carbonate and dilute hydrochloric acid: CuCO3(s) + 2HCl(aq) → CuCl2(aq) + CO2(g) + H2O(l) What state of matter is carbon dioxide in this reaction?
Medium
- AGas
- BAqueous
- CLiquid
- DSolid
37.A student investigates the rate of reaction between calcium carbonate and hydrochloric acid: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) What type of reaction occurs between calcium carbonate and hydrochloric acid?
Medium
- ANeutralisation
- BCombustion
- CDecomposition
- DEndothermic
38.A company produces tin from an ore called cassiterite, which is mainly tin oxide, SnO2. What is the relative formula mass (Mr) of tin oxide? (Ar: Sn = 119, O = 16)
Medium
- A151
- B135
- C270
- D1904
39.A student set up an experiment to investigate the law of conservation of mass. They measured the mass of beaker A and contents before mixing (127.60 g) and the mass of beaker B and contents before mixing (126.86 g). After mixing the solutions, the mass of beaker A and contents was 153.09 g and the mass of beaker B was 101.37 g. What is the resolution of the balance used in the experiment?
Medium
- A0.01 g
- B0.1 g
- C1.0 g
- D10.0 g
40.Why do the contents of the conical flask lose mass during the investigation?
Medium
- ACarbon dioxide gas escapes from the open flask.
- BThe copper carbonate evaporates.
- CThe hydrochloric acid evaporates.
- DThe reaction absorbs mass from the air.
41.Calculate the relative formula mass (Mr) of calcium carbonate, CaCO3. (Ar: C=12, O=16, Ca=40)
Medium
42.Calculate the percentage by mass of oxygen in calcium carbonate, CaCO3. (Ar: C=12, O=16, Ca=40)
Medium
43.Complete the sentence about the law of conservation of mass.
Easy
In a chemical reaction, the total mass of the ____ is equal to the total mass of the ____.
44.What is the name of the law that states no matter is lost or gained during a chemical reaction?
Medium
45.Calculate the relative formula mass (Mr) of potassium carbonate, K2CO3. (Ar: K=39, C=12, O=16)
Medium
46.Calculate the percentage by mass of nitrogen in ammonium nitrate, NH4NO3. (Ar: N=14, H=1, O=16) Give your answer to the nearest whole number.
Medium
47.Calculate the percentage by mass of oxygen in water, H2O. (Ar: H=1, O=16) Give your answer to 1 decimal place.
Medium
48.Complete the sentences about the reaction between lead nitrate and potassium chromate solutions.
Medium
The reaction between lead nitrate and potassium chromate solutions produces a solid called a ____. If this reaction is carried out in a ____ system, the total mass before and after the reaction will be the same.
49.Complete the sentence about the reaction: 2Mg(s) + O2(g) → 2MgO(s).
Medium
In this reaction, the mass of the reaction flask ____ because oxygen from the air is a ____.
50.What is the name of the law that states that no atoms are lost or made during a chemical reaction?
Easy
51.What is the relative formula mass (Mr) of sodium chloride, NaCl? (Ar: Na=23, Cl=35.5)
Medium
52.Calculate the relative formula mass (Mr) of ammonium sulfate, (NH4)2SO4. (Ar: N = 14, H = 1, S = 32, O = 16)
Medium
53.Explain why the mass of the contents of a conical flask decreases when calcium carbonate reacts with dilute hydrochloric acid in an open flask.
Hard
54.Explain how the reaction Fe + O2 → FeO demonstrates the law of conservation of mass.
Hard
55.A student heats magnesium in an open crucible. The mass of the contents increases. Explain why.
Hard
56.A student tests the law of conservation of mass using the reaction: Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l) The student's results appear to fail to support the law of conservation of mass. Explain why this is so.
Hard
57.Calculate the relative formula mass (Mr) of calcium carbonate, CaCO3. (Ar: C=12, O=16, Ca=40)
Medium
- A90
- B110
- C100
- D120
58.Calculate the percentage by mass of oxygen in calcium carbonate, CaCO3. (Ar: C=12, O=16, Ca=40)
Medium
- A43 %
- B48 %
- C53 %
- D58 %
59.Calculate the relative formula mass (Mr) of potassium carbonate, K2CO3. (Ar: K=39, C=12, O=16)
Medium
- A124
- B166
- C138
- D152
60.Calculate the percentage by mass of nitrogen in ammonium nitrate, NH4NO3. (Ar: N=14, H=1, O=16) Give your answer to the nearest whole number.
Medium
- A35 %
- B39 %
- C43 %
- D31 %
61.Calculate the percentage by mass of oxygen in water, H2O. (Ar: H=1, O=16) Give your answer to 1 decimal place.
Medium
- A80 %
- B106.7 %
- C97.8 %
- D88.9 %
62.Calculate the relative formula mass (Mr) of ammonium sulfate, (NH4)2SO4. (Ar: N = 14, H = 1, S = 32, O = 16)
Medium
- A119
- B158
- C145
- D132

字卡

State the law of conservation of mass.

No atoms are lost or made during a chemical reaction, so the total mass of the products equals the total mass of the reactants.

Why must chemical equations be balanced?

Because the law of conservation of mass means the number of atoms of each element must be the same on both sides of the equation.

In a chemical formula, what does a subscript number after an element tell you?

It shows how many atoms of that element are in the formula. For example, in CO2 the subscript 2 means there are two oxygen atoms.

In a formula with brackets, what does a subscript number after the bracket mean?

It multiplies everything inside the bracket. For example, in Ca(OH)2 there are two oxygen atoms and two hydrogen atoms.

What does a large number in front of a chemical formula in an equation mean?

It multiplies the whole formula. For example, 2H2O means two water molecules, giving four hydrogen atoms and two oxygen atoms in total.

What is relative formula mass (Mr)?

The sum of the relative atomic masses of all the atoms in the numbers shown in the formula.

How do you calculate the relative formula mass of a compound?

Add up the relative atomic masses of all the atoms in the formula, remembering to multiply by any subscript numbers.

Calculate the relative formula mass of water, H2O. (Ar: H = 1, O = 16)

(2 × 1) + 16 = 18

Calculate the relative formula mass of calcium hydroxide, Ca(OH)2. (Ar: Ca = 40, O = 16, H = 1)

40 + (2 × 16) + (2 × 1) = 74

Give the equation for calculating the percentage by mass of an element in a compound.

Percentage by mass = (Ar × number of atoms of the element) ÷ Mr of the compound × 100

Calculate the percentage by mass of oxygen in water, H2O. (Ar: H = 1, O = 16)

(16 ÷ 18) × 100 = 88.9%

Why does the mass of a reaction mixture decrease when a gas is produced in an open flask?

The gas escapes into the atmosphere, so its mass is no longer included in the mass of the reaction mixture.

When a metal reacts with oxygen in an open container, why does the mass of the product appear to be greater than the mass of the metal?

Oxygen from the air joins with the metal to form the oxide, so the product has more mass than the metal alone.

Why does the mass of a metal carbonate decrease when it is heated in an open container?

Carbon dioxide gas is produced and escapes into the atmosphere, leaving the solid metal oxide behind.

What is a closed system in a chemical reaction?

A system where no substances can enter or leave, so the total mass stays constant during the reaction.

What is the difference between random error and systematic error?

Random error causes results to vary unpredictably above or below the true value; systematic error always shifts results in the same direction.

How do you estimate the uncertainty from repeated measurements?

Uncertainty is ± half the range of the results. Calculate the range (highest minus lowest) and divide by 2.

What is the uncertainty of a digital balance that reads to 0.01 g?

± 0.01 g

What is the uncertainty of an analogue thermometer that reads to 1 °C?

± 0.5 °C

Balance the equation: __Al + __CuO → __Al2O3 + __Cu

2Al + 3CuO → Al2O3 + 3Cu

Balance the equation: __MgO + __HNO3 → __Mg(NO3)2 + __H2O

MgO + 2HNO3 → Mg(NO3)2 + H2O

In a balanced equation, what is true about the sum of relative formula masses of reactants and products?

The sum of the relative formula masses of the reactants equals the sum of the relative formula masses of the products, in the quantities shown.

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