Question $$ 4 \quad[20 $$ marks $$ ] $$ $$ 4.1 \quad $$ Write an equation in which $$ \mathrm{H}_{2} \mathrm{PO}_{3}^{-} $$acts as an acid and another in which it acts as a base. (2) 4.2 A shampoo solution at $$ 25^{\circ} \mathrm{C} $$ has a hydroxide ion concentration of $$ 1.5 imes 10^{-9} \mathrm{M} $$. 4.2 .1 Calculate its hydronium ion concentration. 4.2 .2 Is this solution acidic, neutral, or basic? Give a reason for your answer. 4.3 A 1.00 L aqueous solution contained 5.80 g of sodium hydroxide, $$ \mathrm{NaOH} . \mathrm{Calculate} $$ the pH of this solution at $$ 25^{\circ} \mathrm{C} $$ ?

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Question $$ 4 \quad[20 $$ marks $$ ] $$ $$ 4.1 \quad $$ Write an equation in which $$ \mathrm{H}_{2} \mathrm{PO}_{3}^{-} $$acts as an acid and another in which it acts as a base. (2) 4.2 A shampoo solution at $$ 25^{\circ} \mathrm{C} $$ has a hydroxide ion concentration of $$ 1.5 	imes 10^{-9} \mathrm{M} $$. 4.2 .1 Calculate its hydronium ion concentration. 4.2 .2 Is this solution acidic, neutral, or basic? Give a reason for your answer. 4.3 A 1.00 L aqueous solution contained 5.80 g of sodium hydroxide, $$ \mathrm{NaOH} . \mathrm{Calculate} $$ the pH of this solution at $$ 25^{\circ} \mathrm{C} $$ ?

Martin Wells · Accepted Answer

### 4.1 **Acidic Reaction:** $$ \mathrm{H}_{2}\mathrm{PO}_{3}^{-} + \mathrm{H}_2\mathrm{O} ightarrow \mathrm{HPO}_{3}^{2-} + \mathrm{H}_3\mathrm{O}^{+} $$ **Basic Reaction:** $$ \mathrm{H}_{2}\mathrm{PO}_{3}^{-} + \mathrm{H}^{+} ightarrow \mathrm{H}_{3}\mathrm{PO}_{3} $$ ### 4.2 **4.2.1 Hydronium ion concentration:** $$ 6.67 	imes 10^{-6} \mathrm{M} $$ **4.2.2 Solution is acidic because $$ [\mathrm{H}_3\mathrm{O}^+] > [\mathrm{OH}^-] $$. ### 4.3 The pH of the solution is approximately **13.16**.

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