Phosphorus pentoxide reacts with water to form phosphoric acid according to the following equation. $$P_2O_5(s) + 3H_2O(l) \rightarrow 2H_3PO_4(aq)$$ Phosphoric acid reacts with sodium hydroxide according to the following equation. $$H_3PO_4(aq) + 3NaOH(aq) \rightarrow Na_3PO_4(aq) + 3H_2O(l)$$ A student reacted 1.42 g of phosphorus pentoxide with excess water. What volume of 0.30 mol L$^{-1}$ sodium hydroxide would be required to neutralise all the phosphoric acid produced?

SSCE HSC Chemistry Chemical Reactions and Stoichiometry: Phosphorus pentoxide reacts with

Phosphorus pentoxide reacts with water to form phosphoric acid according to the following equation - HSC - SSCE Chemistry - Question 10 - 2006 - Paper 1

Question 10

Phosphorus pentoxide reacts with water to form phosphoric acid according to the following equation. $$P_2O_5(s) + 3H_2O(l) \rightarrow 2H_3PO_4(aq)$$ Phosphoric ac... show full transcript

Worked Solution & Example Answer:Phosphorus pentoxide reacts with water to form phosphoric acid according to the following equation - HSC - SSCE Chemistry - Question 10 - 2006 - Paper 1

Step 1

Calculate moles of phosphorus pentoxide

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Answer

To find the number of moles of phosphorus pentoxide (P₂O₅), we use the formula:

$n = \frac{m}{M}$

Where:

$n$ = number of moles
$m$ = mass (1.42 g)
$M$ = molar mass of P₂O₅ ( $= 2 \times 31.00 + 5 \times 16.00 = 141.94 g mol^{-1}$ )

Thus,

$n = \frac{1.42}{141.94} \approx 0.0100 \text{ mol}$

Step 2

Determine moles of phosphoric acid produced

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Answer

From the balanced chemical equation, 1 mol of P₂O₅ produces 2 mol of H₃PO₄. Hence:

$\text{Moles of } H_3PO_4 = 2 \times 0.0100 \approx 0.0200 \text{ mol}$

Step 3

Calculate moles of sodium hydroxide needed

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Answer

From the equation, 1 mol of H₃PO₄ reacts with 3 mol of NaOH. Therefore, the moles of NaOH required are:

$\text{Moles of } NaOH = 3 \times 0.0200 = 0.0600 \text{ mol}$

Step 4

Calculate the volume of sodium hydroxide solution needed

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Answer

Using the concentration of NaOH ( $C = 0.30 \, \text{mol L}^{-1}$ ), we can find the volume using:

$C = \frac{n}{V}$

Where $V$ is the volume in liters. Rearranging gives:

$V = \frac{n}{C} = \frac{0.0600}{0.30} = 0.20 \, \text{L}$

Phosphorus pentoxide reacts with water to form phosphoric acid according to the following equation - HSC - SSCE Chemistry - Question 10 - 2006 - Paper 1

Worked Solution & Example Answer:Phosphorus pentoxide reacts with water to form phosphoric acid according to the following equation - HSC - SSCE Chemistry - Question 10 - 2006 - Paper 1

Calculate moles of phosphorus pentoxide

Determine moles of phosphoric acid produced

Calculate moles of sodium hydroxide needed

Calculate the volume of sodium hydroxide solution needed

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