Reacting Masses

Question 1

In the equation 2Mg + O₂ → 2MgO, the mole ratio of Mg to MgO is:

Accepted Answer

1 : 1 — The coefficients are 2 : 2, which simplifies to 1 : 1.

Question 2

What mass of MgO forms from 12 g of Mg? (Mg=24, MgO=40; 2Mg+O₂→2MgO)

Accepted Answer

20 g — Mol Mg = 12÷24 = 0.5; ratio 1:1 so 0.5 mol MgO; mass = 0.5×40 = 20 g.

Question 3

Reacting mass calculations depend on which law?

Accepted Answer

Conservation of mass — Total mass of reactants equals total mass of products, so masses can be tracked through the ratio.

Question 4

The numbers in a balanced equation represent ratios of:

Accepted Answer

Moles — Coefficients give the ratio of moles, which is why masses are converted to moles first.

Question 5

For CaCO₃ → CaO + CO₂, how many moles of CO₂ come from 2 mol CaCO₃?

Accepted Answer

2 mol — The ratio CaCO₃ : CO₂ is 1 : 1, so 2 mol CaCO₃ gives 2 mol CO₂.

Question 6

What is the first step in a reacting-mass calculation?

Accepted Answer

Convert the known mass to moles — You must convert to moles (n = m ÷ Mr) before using the mole ratio.

Question 7

What mass of carbon dioxide is made when 100 g CaCO₃ decomposes? (CaCO₃=100, CO₂=44)

Accepted Answer

44 g — Mol CaCO₃ = 100÷100 = 1; ratio 1:1 gives 1 mol CO₂; mass = 1×44 = 44 g.

Question 8

For 2H₂ + O₂ → 2H₂O, how many moles of water form from 4 mol H₂?

Accepted Answer

4 mol — Ratio H₂ : H₂O = 2 : 2 = 1 : 1, so 4 mol H₂ gives 4 mol H₂O.

Question 9

What mass of oxygen reacts with 12 g Mg to form 20 g MgO?

Accepted Answer

8 g — By conservation of mass, mass O₂ = 20 − 12 = 8 g.

Question 10

How many moles are in 8 g of oxygen gas O₂? (O₂ = 32 g/mol)

Accepted Answer

0.25 mol — n = m ÷ M = 8 ÷ 32 = 0.25 mol.