U1, S2 Amount of substance

Question 1

1) In the ideal gas equation, what should the units of pressure be?

Answer 1

1) Pa (pascals)

Question 2

2) What does Avogadro’s constant represent?

Answer 2

the number of particles in one mole of a substance: 6.02 x 10²³

Question 3

3) How many atoms are there in 0.500 moles of pure krypton?

Answer 3

3) 3.01 x 10²³.

Number of atoms = moles x Avogadro’s constant
= 0.500 × (6.02 x 10²³) 3.01 x 10²³

Question 4

4) How many moles of scandium fluoride are there in 20.4 g of ScF₃? M_r of ScF₃ = 102.0.

Answer 4

4) 0.200 moles. Number of moles of ScF₃

= mass/M_r = 20.4/ 102.0 =0.200 moles (3 s.f.)

Question 5

5) What mass of selenium dioxide (SeO₂) needs to be dissolved in water to give 40.0 cm³ of a solution with a concentration of 0.500moldm^-³? M_r of SeO₂ = 111.0.

Answer 5

5) 2.22 g.

Volume of solution in dm³ = 40.0 ÷ 1000 = 0.0400 dm³

Number of moles = concentration x volume

= 0.500 x 0.0400 = 0.0200 moles
Mass of SeO₂ = moles x M_r = 0.0200 x 111.0 = 2.22 g

Question 6

6) At 200 kPa, 2.00 moles of a gas occupy a volume of 40.0 dm³.
Calculate the temperature of the gas sample.

Gas constant, R = 8.31 J K^-¹ mol^-¹.

Answer 6

Question 7

7) What volume of water, in cm³, does 12.75 g of sodium nitrate (NaNO₃) need to be dissolved in
to give a solution with a concentration of 1.50 mol dm^-³? M_r of NaNO₃ = 85.0.

Answer 7

Question 8

8) At 37 °C and 100 kPa, a sample of gas with an M_r of 34.0 occupies a volume of 760 cm³.

Calculate the mass of the gas sample. Gas constant, R = 8.31 J K^-¹ mol^-¹.

Answer 8

Question 9

1) What should the state symbol be for the lead iodide precipitate (Pbl₂) in the following equation?
Pb(NO₃)₂ + 2KI → Pbl₂ + 2KNO₃

Answer 9

1) (s)- it’s a precipitate, which means it must be solid.

Question 10

2) Which of the following equations is balanced correctly? A. MnO₂ + 2KOH + O₂ → K₂MnO, + 2H₂O

B. 2PbS+ 2Na₂CO₃ + C → 2Pb + 2Na₂S + 3CO₂

Answer 10

2) B (lf, by chance, you had a wave of equation-balancing enthusiasm and felt keen enough to fix equation A_r the correctly balanced equation would be:

2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O.)

Question 11

3) What does an ionic equation show?

Answer 11

3) An ionic equation shows only the reacting particles for a reaction happening in solution, and the products that they form.

Question 12

4) The Haber process is used to make ammonia (NH₃) from nitrogen and hydrogen gas.
Give the balanced equation for this reaction.

Answer 12

4) N₂ + 3H₂ → 2NH₃

Question 13

5) Balance the following equation: BeO + C → Be₂C + CO

Answer 13

5) 2BeO + 3C → Be₂C + 2CO

Question 14

6) Potassium chloride solution reacts with silver nitrate solution to form potassium nitrate in solution and a silver chloride precipitate. Give the balanced ionic equation for this reaction.
The equation for this reaction is: KCI + AgNO₃ → KNO₃ + AgCl

Answer 14

Question 15

7) Calcium reacts with nitric acid (HNO₃) to produce a solution of calcium nitrate, Ca(NO₃)₂, and hydrogen gas. The equation for this reaction is: Ca + 2HNO_{3 (aq)} →Ca(NO₃)_2(aq) + H₂ _(g) Give the ionic equation for this reaction.

Answer 15

Question 16

1) The neutralisation reaction of magnesium hydroxide with hydrochloric acid produces water.
What is the state symbol of water in the equation for this reaction?

Answer 16

1) (l) - water is a liquid. Don’t go thinking water has the symbol (aq) that’s used for a solution in water, and water in water would just be silly.

Question 17

2) Calculate the mass of potassium chloride (Mr = 74.6) produced when 0.250 moles of potassium chlorate, KCIO₃, fully decomposes in the following reaction: 2KCIO₃ → 2KCI + 30₂

Answer 17

2) 18.7 g. From the equation, 2 moles of KCIO₃ produce 2 moles of KCI, so 0.250 moles of KCIO₃ produce 0.250 moles of KCI. Mass of KCI produced

= moles x Mr

= 0.250 x 74.6

= 18.65 = 18.7 g (3 s.f.)

Question 18

Answer 18

Question 19

Answer 19

Question 20

5) 6.94 g of lithium nitride (Li,N) is made from the reaction of lithium and nitrogen: Li + N₂ → Li₃N
Balance the equation for this reaction, and calculate the mass of lithium that has reacted.

Answer 20

Question 21

1) Why can’t universal indicator be used as an indicator for a titration?

Answer 21

1) E.g. the colour change is too gradual / there’s no clear end point.

Question 22

2) What is the purpose of a rough titration?

Answer 22

2) It gives an idea of where the end point of the titration is, before any accurate titrations are carried out.

Question 23

3) How many grams of sodium carbonate (Na₂CO₃) are needed to make 200 cm³ of 1.50 mol dm³ sodium carbonate standard solution? Mr of Na₂CO₃ =
106. 0.

Answer 23

Question 24

4) The results for a titration are shown in the table on the right.
State which result is anomalous, and explain your reasoning.

5) Suggest a possible reason for the anomalous result.
6) Calculate the mean titre of the results, ignoring the anomalous result.

Answer 24

4) Titration 2 is anomalous as it is significantly different to the others.

5) E.g. the solution in the burette wasn’t added dropwise around the end point, meaning the exact point at which the colour changed was missed / wasn’t recorded accurately.

Question 25

Answer 25

Question 26

1) You should wear safety glasses when carrying out a titration. Give an example of another safety precaution that you should take.

Answer 26

1) E.g. pour acid/alkali into the burette below eye level to avoid any solution splashing on to your face or eyes.

Question 27

2) Describe the difference between the uses of a pipette and a burette.

Answer 27

2) A pipette measures one volume of a solution and is used to measure out a set volume of the solution into a flask, whereas a burette measures different volumes of a solution, and can be used to add the solution drop by drop.

Question 28

3) A student wants has to carry out a titration to find out how much alkali of a known concentration is needed to neutralise a known volume of acid. Her set-up is shown
on the right. State which of the labels A and B is the acid, and which is the alkali.

Answer 28

3) The solution in the burette, A, should be the alkali, because that is the solution of known concentration. The solution in the flask, B, should be the acid, because that is the solution with the unknown concentration.

Question 29

4) Outline how the student should carry out an accurate titration to find out how much alkali is needed to neutralise the acid.

Answer 29

4) E.g. use a pipette to measure out a set volume of the acid into a conical flask. Add a few drops of indicator
to the flask. Use a funnel to pour the standard solution of alkali into a burette. Take an initial reading of how
much alkali is in the burette. Then run the alkali into the acid until you get to within 2 cm³ of the end point,
giving the flask a regular swirl. At this point, add the alkali dropwise, so that you don’t miss exactly when the
colour changes and overshoot the end point. Work out the amount of alkali used to neutralise the acid (final
reading minus the initial reading). Repeat the titration a few times, until you have at least three concordant results. Use the results to find the mean volume of alkali used.

Question 30

Answer 30

Question 31

1) What is meant by the term ‘molecular formula’?

Answer 31

1) A formula that gives you the actual numbers of atoms of each element in a compound.

Question 32

2) A molecule with Mr =90.0 has the empirical formula CHO₂. What is its molecular formula?

Answer 32

Question 33

3) The percentage composition by mass of compound W is shown in
the table below. Find the empirical formula of compound W.

Answer 33

Question 34

4) Compound X only contains nitrogen and hydrogen atoms. A sample of
compound X is burnt in excess oxygen and makes 2.8 g of nitrogen (N₂)
and 5.4 g of water (H₂O). Find the empirical formula of compound X.

Answer 34

Question 35

5) Compound Y has a relative molecular mass of 254.2 and a percentage composition of 25.3% sulfur and 74.7% fluorine by mass.

6) Use your answer to Q5 to find the molecular formula of compound Y.
Find the empirical formula of compound Y.

Answer 35

6) S₂F₁₀ Empirical mass

= 32.1 + (19.0 x 5) = 127.1 254.2 = 127.1

= 2 empirical units in the molecule, so the molecular formula is S₂F₁₀.

Question 36

1) What is meant by the term ‘theoretical yield”?

Answer 36

1) The mass of product that should be formed in a chemical reaction.

Question 37

Answer 37

Question 38

Answer 38

Question 39

4) Chromium(III) oxide reacts with magnesium oxide and oxygen to produce magnesium chromate
as the only product. What is the % atom economy for this reaction? Explain your answer.

Answer 39

4) The% atom economy is 100% all of the reactant atoms become part of the desired product as there is only one product.

Question 40

Answer 40

Question 41

6) Two reactions are used to produce the same desired product.
Some information about each reaction is shown in the table.
Why might a company use reaction B even though it has a lower yield?

Answer 41

6) Reaction B has a higher atom economy, meaning less waste is produced, which is better for the environment. Reactions with higher atom economies are more sustainable because they make more efficient use of raw materials. They’re less expensive as less money has to be spent on separating the desired product from waste products, and on treating waste.