**Achieved**1 Calculate the amount (in moles) of carbon dioxide, CO2, in 23.0 g of the gas?

M(CO2) = 44.0 g mol-1

**n(CO2) = = 0.523 mol****Achieved**2 Calculate the mass of 2.65 mol of sodium hydroxide, NaOH.

M(NaOH) = 40.0 g mol-1

**m = 2.65 mol x 40.0 g mol–1 = 106 g****Achieved**3 24.5 mL of a solution contains 0.00257 mol of HCl. Calculate the concentration, in mol L–1, of the HCl solution.

**c = = 0.105 mol L–1****Achieved**4 Calculate the amount (in mole) of NaOH present in 15.6 mL of 0.152 mol L–1 solution.

n = 0.152 mol L–1 x 15.6 x 10–3

**L = 2.37 x 10–3 mol****Merit**5(a) Caffeine is a stimulant which has a mass composition of 49.5% carbon, 5.20% hydrogen, 28.9% nitrogen and 16.5% oxygen.

Calculate the empirical formula of caffeine.

(a) n(C) = = 4.13 mol n(H) = = 5.20 mol

n(N) = = 2.06 mol n(O) = = 1.03 mol

**empirical formula C4H5N2O **Amount in moles of each element correct.

Correct process, but incorrect empirical formula used. (achieved)

(b) If the molar mass of caffeine is 194 g mol-1, use your answer to part (a) above to determine the molecular formula of caffeine.

**molecular formula C8H10N4O2**

Empirical and molecular formulae correct. (merit)**Merit**6. Calculate the percentage of carbon and oxygen in sucrose, C12H22O11.

M(C12H22O11) = 342 g mol–1

% C= x 100 = **42.1 %**

% O = x 100 = **51.5 %**

Process correct but incorrect molar mass of sucrose used. (Achieved)

% of C and O correct(Merit)

**Merit**7. 500 mL of 0.253 mol L-1 NaHCO3 solution is mixed with 800 mL of 0.824 mol L-1 NaHCO3 solution.

What is the concentration of the final solution?

n(NaHCO3)

= (0.253 mol L-1 x 0.500 L) + (0.824 mol L-1 x 0.800 L)

= 0.786 mol

Total volume = 1.30 L

concentration of final solution =** = 0.604 mol L-1**

Correct total amount of NaHCO3

OR

correct process for calculating concentration using a volume of 1.3 L. (Achieved)

Correct determination of concentration of final solution. (Merit)

**Excellence**8. A chemist found that 4.69 g of sulfur combined with fluorine t

o produce 15.81 g of gas. Determine the empirical formula of the compound.

n(S) = = 0.146 mol

m(F) = 15.81 - 4.69 = 11.1 g

n(F) = = 0.584 mol

**Hence S : F = 1 : 4 and empirical formula is SF4.**

Correct amount of S OR F. (achieved)

Correct process with one minor error. eg Incorrect mass of F (Merit)

Correct determination empirical formula. (excellence)

**Excellence**9. What mass of CO2 is produced in the complete combustion of 36.5 g of ethanol according to the following equation?

C2H5OH + 3 O2 2 CO2 + 3 H2O

M(C2H5OH) = 46.0 g mol-1

(i) n(C2H5OH) = 36.5 g / 46.0 g mol-1 = 0.793 mol

(ii) n(CO2) = 2 x n(C2H5OH) = 1.59 mol

(iii) m(CO2) = 1.59 mol x 44.0 g mol-1 **= 69.8 g**

Correct process used in either step (i) or (iii). (achieved)

Correct process with one minor error. eg An incorrect molar mass. (Merit)

Correct answer for mass of CO2 produced. (excellence)

**Excellence**10 .What mass of iron can be produced if 50.0 g of carbon monoxide react with iron(III) oxide according to the following equation?

Fe2O3 + 3 CO 2 Fe + 3 CO2

(i) n(CO) = = 1.79 mol

(ii) n(Fe) = x n(CO) = 1.19 mol

(iii) m(Fe) = 1.19 mol x 55.9 g mol-1 **= 66.5 g**

Correct process used in either step (i) or (iii).(achieved)

Correct process with one minor error.eg An incorrect molar mass.(Merit)

Correct answer for mass of Fe produced. (Excellence)

**Excellence**11. Hydrated magnesium sulfate is heated in a crucible. The following data is collected:

mass of crucible and lid = 26.49 g

mass of hydrated magnesium sulfate = 2.13 g

mass crucible, lid and magnesium sulfate after first heating = 27.55 g

mass of crucible, lid and magnesium sulfate after second heating = 27.53 g

Use these results to determine the formula of hydrated magnesium sulfate.

M(MgSO4) = 120.3 g mol-1 M(H2O) = 18.0 g mol-1

mass of anhydrous MgSO4 = 1.04 g

mass of H2O = 1.09 g

amount of MgSO4 = = 8.65 x 10-3 mol

amount H2O = = 6.06 x 10-2 mol

ratio n(H2O) / n(MgSO4) = 7

**formula is MgSO4.7H2O**

Correct determination of masses of MgSO4 and H2O.(Achieved)

Correct determination of either amount of MgSO4 or H2O. (Merit)

Correct formula for hydrated salt.(Excellence)