Zinc metal reacts with hydrochloric acid according to the balanced chemical equation: \text{Zn}_{(s)} + 2\text{HCl}_{(aq)} \rightarrow \text{ZnCl}_{2(aq)} + \text{H}_{2(g)} If 13.0\text{ g} of zinc is added to a solution containing 7.3\text{ g} of hydrochloric acid, what mass of zinc remains unreact

Zinc metal reacts with hydrochloric acid according to the balanced chemical equation: \tex… — JAMB UTME

Zinc metal reacts with hydrochloric acid according to the balanced chemical equation:

\text{Zn}_{(s)} + 2\text{HCl}_{(aq)} \rightarrow \text{ZnCl}_{2(aq)} + \text{H}_{2(g)}

13.0\text{ g}

of zinc is added to a solution containing

7.3\text{ g}

of hydrochloric acid, what mass of zinc remains unreacted after the reaction goes to completion?
(

\text{Zn} = 65.0,\text{ H} = 1.0,\text{ Cl} = 35.5

)

$6.5\text{ g}$ Answer
B
$5.7\text{ g}$
C
$0.0\text{ g}$
D
$3.25\text{ g}$

Answer

The mass of zinc remaining unreacted is

6.5\text{ g}

Converting the given masses into moles shows that 0.20 mol of zinc and 0.20 mol of hydrochloric acid are present. Since 1 mole of zinc requires 2 moles of hydrochloric acid, 0.20 mol of hydrochloric acid reacts with only 0.10 mol of zinc. Hydrochloric acid is completely consumed, leaving 0.10 mol (6.5 g) of zinc unreacted.

Step-by-Step Solution

Calculate the molar masses of the reactants

Molar mass of

\text{Zn} = 65.0\text{ g/mol}

; Molar mass of

\text{HCl} = 1.0 + 35.5 = 36.5\text{ g/mol}

Molar masses are required to convert the given masses to mole quantities.

Determine the initial mole quantities of each reactant

Moles of

\text{Zn} = \frac{13.0\text{ g}}{65.0\text{ g/mol}} = 0.20\text{ mol}

; Moles of

\text{HCl} = \frac{7.3\text{ g}}{36.5\text{ g/mol}} = 0.20\text{ mol}

Chemical reactions occur according to mole ratios, not mass ratios.

Identify the limiting reactant and calculate the moles of zinc consumed

From the equation,

1\text{ mol of Zn}

reacts with

2\text{ mol of HCl}

. Thus,

0.20\text{ mol of HCl}

requires

\frac{0.20}{2} = 0.10\text{ mol of Zn}

\text{HCl}

is the limiting reactant.

The limiting reactant determines the extent of the reaction.

Calculate the unreacted moles and mass of zinc

Unreacted moles of

\text{Zn} = 0.20\text{ mol} - 0.10\text{ mol} = 0.10\text{ mol}

. Unreacted mass of

\text{Zn} = 0.10\text{ mol} \times 65.0\text{ g/mol} = 6.5\text{ g}

Subtracting consumed moles from initial moles gives the remaining amount.

Key Concept

Limiting and Excess Reactants

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