How much did you really make? — questions

1. calculate

Supplement. A reaction should make 2.00 g of product, but 1.70 g is collected. What is the percentage yield? Give your answer to 3 significant figures.

Percentage yield: ______________ unit ________

Nudge (only if you are stuck): percentage yield = actual yield ÷ theoretical yield × 100.

2. calculate

Supplement. Calculate the percentage by mass of (a) oxygen in water, H₂O, and (b) calcium in calcium carbonate, CaCO₃. (Ar: H 1, C 12, O 16, Ca 40.) Give your answers to 3 significant figures.

(a) Oxygen in water: ______________ unit ________

(b) Calcium in calcium carbonate: ______________ unit ________

Nudge (only if you are stuck): % of an element = atoms × Ar ÷ Mr × 100.

3. state

Supplement. What is an empirical formula?

Circle one answer.

• The simplest whole-number ratio of the atoms of each element in a compound
• The formula with the largest whole numbers
• The ratio of the masses of the elements in a compound
• The actual number of atoms of each element in one molecule

Nudge (only if you are stuck): Think of MgO from the masses in the reading.

4. calculate

Supplement. 4.00 g of copper(II) oxide is reduced to copper: CuO + H₂ → Cu + H₂O. (a) What is the theoretical yield of copper? (b) 2.88 g of copper is collected. What is the percentage yield? (c) What is the percentage by mass of copper in copper(II) oxide? (Ar: O 16, Cu 64.) Give your answers to 3 significant figures.

(a) Theoretical yield of copper: ______________ unit ________

(b) Percentage yield: ______________ unit ________

(c) Copper in copper(II) oxide: ______________ unit ________

Nudge (only if you are stuck): The theoretical yield comes from the equation, in three steps.

5. calculate

Supplement. A 5.00 g sample of impure calcium carbonate reacts with excess acid and gives 0.960 dm³ of carbon dioxide at r.t.p.: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The impurities make no gas. (a) What mass of calcium carbonate was in the sample? (b) What is its percentage purity? (Ar: C 12, O 16, Ca 40.) Give your answers to 3 significant figures.

(a) Mass of calcium carbonate: ______________ unit ________

(b) Percentage purity: ______________ unit ________

Nudge (only if you are stuck): Use the gas to find the moles of CaCO₃.

6. calculate / deduce

Supplement. A compound of carbon and hydrogen contains 2.40 g of carbon and 0.600 g of hydrogen. (a) How many moles of carbon atoms does it contain? (b) How many moles of hydrogen atoms? (c) What is its empirical formula? (Ar: H 1, C 12.) Give your answers to 3 significant figures.

(a) Carbon atoms: ______________ unit ________

(b) Hydrogen atoms: ______________ unit ________

(c) The empirical formula, and how you found it.

Nudge (only if you are stuck): Divide each mass by its Ar, then by the smaller answer.

7. deduce

Supplement. An oxide of iron contains 11.2 g of iron and 4.80 g of oxygen. What is its empirical formula? (Ar: O 16, Fe 56.)

Circle one answer.

• Fe₃O₄
• FeO₂
• Fe₂O₃
• FeO

Show the ratio you found.

Nudge (only if you are stuck): Divide by Ar, then by the smallest. Is the ratio whole?

8. deduce

Supplement. A hydrocarbon has the empirical formula CH₂ and Mr = 42. What is its molecular formula? (Ar: H 1, C 12.)

Circle one answer.

• C₆H₁₂
• C₃H₆
• CH₂
• C₂H₄

Show how many CH₂ units are in one molecule.

Nudge (only if you are stuck): How many CH₂ units make 42?

9. explain / suggest

Supplement. A pupil makes a salt and gets a percentage yield of 78 %. (a) Give two reasons why the yield is below 100 %. (b) Another pupil gets a yield of 112 %. Suggest why.

(a) Two reasons for a yield below 100 %

(b) Why a yield of 112 %

Nudge (only if you are stuck): Where could some product be lost? What could make the product too heavy?

10. calculate / deduce

Supplement. Butane, the fuel in camping gas, contains 82.8 % carbon and 17.2 % hydrogen by mass. Its Mr is 58. (a) How many moles of carbon atoms and (b) of hydrogen atoms are in 100 g? (c) How many empirical-formula units are in one molecule? (d) Write its empirical formula and its molecular formula. (Ar: H 1, C 12.) Give your answers to 3 significant figures.

(a) Carbon atoms in 100 g: ______________ unit ________

(b) Hydrogen atoms in 100 g: ______________ unit ________

(c) Empirical-formula units in one molecule: ______________ unit ________

(d) The empirical formula and the molecular formula, with your working.

Nudge (only if you are stuck): Treat the percentages as grams in 100 g. Is the ratio whole?

11. calculate

Supplement. Aspirin, C₉H₈O₄, is made from salicylic acid, C₇H₆O₃: C₇H₆O₃ + C₄H₆O₃ → C₉H₈O₄ + CH₃COOH. A trial batch starts with 13.8 g of salicylic acid and gives 15.3 g of dry aspirin. (a) What is the theoretical yield? (b) What is the percentage yield? (c) A finished tablet has a mass of 0.325 g, and 0.300 g of it is aspirin. What is the percentage purity of the tablet? (Ar: H 1, C 12, O 16.) Give your answers to 3 significant figures.

(a) Theoretical yield of aspirin: ______________ unit ________

(b) Percentage yield: ______________ unit ________

(c) Percentage purity of the tablet: ______________ unit ________

Nudge (only if you are stuck): The theoretical yield comes from the equation. Purity compares the aspirin with the whole tablet.