When does the acid run out? — questions

1. state

Supplement. What is the limiting reactant in a reaction?

Circle one answer.

• The reactant with fewer moles
• The reactant with the smaller mass
• The reactant that is used up completely, so it decides how much product forms
• The reactant that is left over at the end

Nudge (only if you are stuck): Which reactant decides when the reaction stops?

2. deduce

Supplement. In the 0.100 g run of the investigation, the 100 cm³ gas syringe stops at 60 cm³ and pieces of magnesium are left in the flask. Which statement explains this?

Circle one answer.

• The magnesium has run out: it is the limiting reactant
• The acid has run out: it is the limiting reactant, and the magnesium is in excess
• Both reactants have run out together
• The syringe is full, so no more gas can go in

Nudge (only if you are stuck): What is still in the flask, and what has gone?

3. calculate / deduce

Supplement. 2.80 g of iron is heated with 2.00 g of sulfur: Fe + S → FeS. (a) How many moles of iron are there? (b) How many moles of sulfur? (c) What mass of iron(II) sulfide forms? (Ar: S 32, Fe 56.) Give your answers to 3 significant figures.

(a) Amount of iron: ______________ unit ________

(b) Amount of sulfur: ______________ unit ________

(c) Mass of iron(II) sulfide: ______________ unit ________

Which reactant is limiting? Say how you know.

Nudge (only if you are stuck): Both numbers in the equation are 1, so the smaller amount is limiting.

4. calculate / deduce

Supplement. 0.480 g of magnesium is added to 25.0 cm³ of 1.00 mol/dm³ hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. (a) Which reactant is limiting? (b) What volume of hydrogen forms, in cm³ at r.t.p.? (c) What is the concentration of magnesium chloride in the solution at the end, in mol/dm³? Take the volume of the solution as 25.0 cm³. (Ar: Mg 24.) Give your answers to 3 significant figures.

(b) Volume of hydrogen: ______________ unit ________

(c) Concentration of magnesium chloride: ______________ unit ________

(a) Which reactant is limiting? Show how you decided.

Nudge (only if you are stuck): Find both amounts in moles, then divide each by its number in the equation.

5. calculate

Supplement. 0.300 g of magnesium is added to 20.0 cm³ of 0.500 mol/dm³ hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂. When the fizzing stops, what mass of magnesium is left? (Ar: Mg 24.) Give your answer to 3 significant figures.

Mass of magnesium left: ______________ unit ________

Nudge (only if you are stuck): How much magnesium can the acid react with?

6. calculate

Supplement. 20.0 cm³ of 0.100 mol/dm³ barium chloride solution is mixed with 10.0 cm³ of 0.100 mol/dm³ sodium sulfate solution. A white precipitate of barium sulfate forms: BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl. What mass of barium sulfate can form? (Ar: O 16, S 32, Ba 137.) Give your answer to 3 significant figures.

Mass of barium sulfate: ______________ unit ________

Nudge (only if you are stuck): n = c × V for each solution, with V in dm³.

7. explain

Supplement. To make copper(II) sulfate, a chemist adds black copper(II) oxide to warm sulfuric acid until no more dissolves, so some black solid is left over. (a) Which reactant is in excess? (b) Explain why the chemist uses an excess of it.

(a) The reactant in excess

(b) Why use an excess

Nudge (only if you are stuck): Which reactant is left over, and what does that make sure of?

8. calculate / deduce

Supplement. Nitrogen and hydrogen make ammonia: N₂ + 3H₂ → 2NH₃. 100 cm³ of nitrogen is mixed with 240 cm³ of hydrogen. Imagine the reaction goes on until one gas runs out. All volumes are at r.t.p. (a) What volume of ammonia forms? (b) What volume of the other gas is left over?

(a) Volume of ammonia: ______________ unit ________

(b) Volume of gas left over: ______________ unit ________

(b) Which gas is left over?

Nudge (only if you are stuck): All three are gases, so the volumes come in the mole ratio 1 : 3 : 2.