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C2.1Relative atomic and molecular masses, the Avogadro constant, and the moleAQA AS Chemistry C2 Amount of Subtance and Kerboodle Answers: Page No. 24
a CH4
16.0
b Na2CO3
106.0
c Mg(OH)2
58.3
d (NH4)2SO4
132.1
Many answers possible such that the relative atomic masses add up to 16, for example. 16 hydrogen atoms; or one carbon atom and 4 hydrogen atoms.
3 Calculate the number of moles in the given masses of the following entities.
a 32.0gCH4
2
b 5.30g Na2CO33
0.05
c 5.83g Mg(OH)2
0.1
4.
4g O2
5
11g CO2
C2.2 Moles in solutions AQA AS Chemistry C2 Amount of Subtance and Kerboodle Answers : Page No. 26
Summary questions
1
1 mol dm-3
b 0.250 mol acid in 2000 cm3 of solution
0.125 mol dm-3
c 0.200 mol solu× 1te in 20 cm3 of solution
10 mol dm-3
2 Calculate how many moles of solute there are in the following.
a 20.0 cm3 of a 0.100 mol drn•3 solution
0.002
b 50.0 cm3 of a 0.500 mol dm-3 solution
0.025
c 25.0 cm3 of a 2.00 mol dm-3 solution
0.05
3 0.234 g of sodium chloride was dissolved in water to make 250 cm3 of solution.
a State the Mr for NaCl.
ArNa= 23.0, Ar Cl= 35.5
58.5
b Calculate how mang moles of NaCl is in 0.234 g.
0.004
c Calculate the concentration in mol dm3
0.016
C2.3 The ideal gas equation AQA AS Chemistry C2 Amount of Subtance and Kerboodle Answers : Page No. 29
Summary Questions
1 © Approximately 8.75 x 106
b The original design used helium. State how many moles of helium atoms it would have contained.
Approximately 8.75 x 106
2.
50360 cm3
b Calculate the pressure of 0.5 moles of a gas if the volume is 11000 cm3, and the temperature is 25 °C.
113000 Pa
3
1.94
4 1.94
The same number or moles of any gas has the same volume under the same conditions of temperature and pressure.
C2.4 Empirical and molecular formulae AQA AS Chemistry C2 Amount of Subtance and Kerboodle Answers: Page No. 34
Summary questions
1
H2SO4 sulfuric acid
b A white solid containing 4.0 g calcium, 3.2 g oxygen, and 0.2 g hydrogen.
Ca(OH)2 calcium hydroxide
c A white solid containing 0.243 g magnesium and 0.?10 g chlorine.
MgCl2 magnesium chloride
2
a Calculate how many moles of magnesium and oxygen are present in 6.448 g Of magnesium oxide.
0.16mol Mg. 0.16 mol O
b State the empirical formula of magnesium oxide.
MgO
3 State the empirical formula of each of the following molecules?
a cyclohexane, C6H12
CH2
bdichloroethene, C2H2C12
CHCI
c benzene, C6H6
CH
4
C2H6O2
5 C3H6O
6
CH
b the molecular formula.
C6H6
C2.5 Balanced equations and related calculationsAQA AS Chemistry C2 Amount of Subtance and Kerboodle Answers: Page No. 38
Summary questions
1
a Mg + O2→MgO
2Mg + 02 → 2Mg0
b Ca(OH)2 +HCl→ CaCl2 + H2O
Ca(OH)2 + 2HC1 → CaCl2 + 2H20
c Na2O+HNO3+ → NaNO3 + H2O
Na2O + 2HNO3→ 2NaNO3 + H2O
2 0.25mol dm-3
3
Yes. there are 0.107 mol Mg. This would be enough to rcac1 with 0.0.214 molHCl. but there is only 0.100 mol HCI.
b 1238 cm3
4
HSO(aq) + 2NaOH(aq) → NaS04 (aq) + 2H20(1)
ii in terms of ions.
2H+ + SO42-+ 2Na++ 2OH–→2Na++ SO42-+ 2H20
b Identify the spectator ions in this reaction.
Na+ and SO42
C2.6 Balanced equations, atom economies, and percentage yields AQA AS Chemistry C2 Amount of Subtance and Kerboodle Answers : Page No. 41
Summary questions
1
Cacooo→Cao + coo
56.0%
2
79.8%
3
100% All the reactants arc incorporated imo the desired product
4 Consider the reaction
CaCO3 → CO2 + CaO
a 1 mol
b 5.6 g
c If 3.6 g of calcium oxide was obtained, calculate the yield of the reaction.
64.3%
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Practice questions: Page No. 42-43
1 Potassium nitrate, KNO3, decomposes on strong heating, forming oxygen and solid Y as the only products.
(a) A 1.00 g sample of KNO3 (M 101.1) was heated strongly until fully decomposed into Y.
(i) Calculate the number of moles of KNOB in the 1.00 g sample.
Moles KNO 3 = 1.00/101.1 = 9.89 × 10-3mol
(ii) At 298 K and kPa, the oxygen gas produced in this decomposition occupied a volume Of 1.22 x 10-4
State the ideal gas equation and use it to calculate the number of moles of oxygen produced in this decomposition.
(The gas constant R 8.31 .JK-I mol-I) (5 marks)
pV = nRT or n = pV / RT
moles O2 = n = pV / RT
= (100 000 × 1.22 × 10–4) / (8.31 × 298)
= 4.93 × 10–3 mol
(b) Compound Y contains 45.9% of potassium and 16.5% of nitrogen by mass, the remainder being oxygen.
(i) State what is meant by the term empirical formula.
Simplest ratio of atoms of each element in a compound.
(ii) Use the data above to calculate the empirical formula of Y. (4 marks)
(c) Deduce an equation for the decomposition of KINO, into Y and oxygen. (1 mark) AQA, 2006
2KNO3 → 2KNO2 + O2
2 Ammonia is used to make nitric acid, HNO3, by the Ostwald Process.
Three reactions occur in this process.
Reaction 1 4NH3(g) + 5O2 (g) → 4NO(g) + 6H2O(g)
Reaction 2 2NO(g) + O2(g)→ 2NO2(g)
Reaction 3 3NO2(g) + 2HNO/(aq) + NO(g)
(a) In one production run, the gases formed in Reaction I occupied a total volume of
4.31 1113 at 25 oc and 100 kPa.
Calculate the amount, in moles, of NO produced.
Give your answer to the appropriate number of significant figures.
(The gas constant R z 8.31 J K -1 mol-1) (4 marks)
P = 100 000 Pa and T = 298 K
n = PV/RT or (100 000 × 4.31) / (8.31 × 298)
n(total) = 174(.044)
n (NO) = 69.6
(b) In another production run, 3.00 kg of ammonia gas were used in Reaction I and all of the NO gas produced was used to make NO2 gas in Reaction 2.
Calculate the mass of NC)2 formed from 3.00 kg of ammonia in Reaction 2 assuming an 80.0% yield.
Give your answer in kilograms. (5 marks)
3000 / 17
176.5
176.47 × 46 = 8117.62
8117.62 × 80 / 100 ( = 6494 g)
6494 / 1000 = 6.5
OR
If 163 mol used: 163 × 46 = 7498 (1)
7498 × 80 / 100 = 5998.4 g (1)
6.00 kg (1)
(c) Consider Reaction 3 in this process.
3NO2 (g) + H2O(1) 2HNO3(aq) + NO(g)
Calculate the concentration of nitric acid produced when 0.543 mol of
N02 is reacted with water and the solution is made up to 250cm3. (2 marks)
0.543 × 2 / 3 ( = 0.362)
0.362 × 1000 / 250 = 1.45 mol dm-3
(d) Suggest why a leak of NO2 gas from the Ostwald Process will cause atmospheric pollution. (1 mark)
NO2 contributes to acid rain / is an acid gas /forms HNO3 / NO2 is toxic / photochemical smog
(e) Give one reason why excess air is used in the Ostwald Process. (1 mark)
Ensure the ammonia is used up / ensure complete reaction or combustion
OR
Maximise the yield of nitric acid or products
(f) Ammonia reacts with nitric acid as shown in this equation.
NH3 + HNO3 -o NH4NO3
Deduce the type of reaction occurring. (1 mark) AQA, 2013
Neutralisation
3 Zinc forms many different salts including zinc sulfate, zinc chloride, and zinc fluoride.
(a) People who have a zinc deficiency can take hydrated zinc sulfate, ZnSO .xH20, as a dietary supplement.
A student heated 4.38 g of hydrated zinc sulfate and obtained 2.46 g of anhydrous zinc sulfate. Use these data to calculate the value of the integer x in ZnS04.xH20.
Show your working. (3 marks)
Method 1
Mass of H2O = 4.38-2.46 = 1.92 g
ZnSO4 H2O
2.46 / 161.5 1.92 / 18(0.0152 0.107)( 1 : 7 )
x = 7
Method 2
Percentage of H2O = 44%
ZnSO4 H2O
56 / 161.5 18 / 44
(0.347 2.444)( 1 : 7 )
x = 7
(b) Zinc chloride can be prepared in the laboratory by the reaction between zinc oxide and hydrochloric acid.
The equation for the reaction is:
ZnO + 2HCl → ZnCl2 + H2O
A 0.0830 mol sample of pure zinc oxide was added to 100 cm3 of 1.20 moldrn-g hydrochloric acid.
Calculate the maximum mass of anhydrous zinc chloride that could be obtained from the products of this reaction. Give your answer to the appropriate number of significant figures. (4 marks)
Moles HCl = 0.12(0)
mol ZnCl2 = 0.06(0) OR 0.12 / 2
mass ZnCl2 = 0.06 × 136.4
= 8.18(4) g OR 8.2 g
(c) Zinc chloride can also be prepared in the laboratory by the reaction between zinc and hydrogen chloride gas.
Zn + 2HCl→ ZnCl2 + H2.
An impure sample of zinc powder with a mass of 5.68 g was reacted with hydrogen chloride gas until the reaction was complete. The zinc chloride produced had a mass of 10.7 g.
Calculate the percentage purity of the zinc metal. Give your answer to 3 significant figures. (4 marks) AQA, 2013
Moles ZnCl2 = 10.7 / 136.4 (= 0.0784)
OR moles Zn = 0.0784
Mass Zn reacting = 0.0784 × 65.4 = 5.13 g
% purity of Zn = 5.13 / 5.68 × 100
= 90.2% OR 90.3%
4 In this question give all your answers to the appropriate number of significant figures.
Magnesium nitrate decomposes on heating to form magnesium oxide, nitrogen dioxide, and oxygen as shown in the following equation.
2Mg(NO3)2 (s) → 2MgO(s) + 4NO2 (g) + O2(g)
(a) Thermal decomposition of a sample of magnesium nitrate produced 0.741 g of magnesium oxide.
(i) Calculate the amount, in moles, of MgO in 0.741 g of magnesium oxide. (2 marks)
MrMgO = 40.3
0.741/40.3 = 0.0184
(ii) Calculate the total amount, in moles, of gas produced from this sample of magnesium nitrate. (1 mark)
0.0184 × 5 / 2 = 0.0460
(b) In another experiment, a different sample of magnesium nitrate decomposed occupy at 333 K and 1.00 x Pa. (The gas constant R 8.31 JK-1 mol-1) (3 marks)
pV=nRT
(V= 0.402 × 8.31 × 333 ) / 100 000
0.0111
11.1 dm3
(c) A 0.0152 mol sample of magnesium oxide, produced from the decomposition of magnesium nitrate, was reacted with hydrochloric acid.
Mgo + 2HCl → MgCl2 + H2O
This 0.0152 mol sample of magnesium oxide required 32.4 cm3; of hydrochloric acid for complete reaction. Use this information to calculate the concentration, in mol dm-3, of the hydrochloric acid. (2 marks) AQA, 2010
0.0152 × 2 = 0.0304
0.938 mol dm-3
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