CAIE A-Level Chemistry 25.1 Acids & bases

[Maximum number: 1]

Hypophosphorous acid is an inorganic acid.
The conjugate base of hypophosphorous acid is $\mathrm{H}_{2} \mathrm{PO}_{2}^{-}$ .

Give the formula of hypophosphorous acid.

$\mathrm{H}_{3} \mathrm{PO}_{2}$

[Maximum number: 15]

(a)

Define conjugate acid-base pair.

[ 1 ]

two species that differ by one proton / $\mathrm{H}^{+}$

(b)

Give the formulas of the conjugate acid and the conjugate base of the hydrogen phosphate ion, $\mathrm{HPO}_{4}{ }^{2-}$ .

\begin{aligned} & \text { conjugate acid of } \mathrm{HPO}_{4}^{2-} \\ & \text { conjugate base of } \mathrm{HPO}_{4}^{2-} \end{aligned}

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$\mathrm{H}_{2} \mathrm{PO}_{4}{ }^{-}$ AND PO4 ${ }^{3-}$

(c)

The $K_{a}$ of propanoic acid, $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ , is $1.35 \times 10^{-5} \mathrm{moldm}^{-3}$ at 298 K.

Solution C is a solution of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ with a pH of 3.60 at 298 K.

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(i)

Calculate the concentration of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ in solution C.

\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right]=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots . . . \mathrm{mol} \mathrm{dm}^{-3}

[2]

[ 2 ]

M1: $\left[\mathrm{H}^{+}\right]=2.51 \times 10^{-4}$

M2: $\left[\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}\right]=4.67 \times 10^{-3}$

(ii)

Calculate the concentration of hydroxide ions in solution C.

\left[\mathrm{OH}^{-}\right]=..............................\ \mathrm{mol\ dm}^{-3}

[ 1 ]

$3.98 \times 10^{-11}$

(iii)

Calculate the concentration of a solution of hydrochloric acid with the same pH as solution C.

\text{concentration}=..............................\ \mathrm{mol\ dm}^{-3}

[ 1 ]

$[\mathrm{HCl}]=2.51 \times 10^{-4}$

(iv)

Table 3.1 shows three possible values of the $K_{a}$ of dimethylpropanoic acid, $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCOOH}$ .

Place a tick in Table 3.1 to show the correct value. Explain your answer.

Table 3.1

explanation

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(d)

Solution D is made by mixing $100 \mathrm{~cm}^{3}$ of $0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ and $100 \mathrm{~cm}^{3}$ of $0.100 \mathrm{~mol} \mathrm{dm}^{-3} \mathrm{NaCl}$ .

The pH of solution D is measured as small amounts of $\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq})$ are added to it, and when small amounts of NaOH(aq) are added to it.

Solution D only acts as a buffer solution when one of these solutions is added to it.

[ 2 ]

(i)

Complete the sentence and write an equation for the reaction that occurs.

Solution D acts as a buffer when is added to it. equation

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NaOH AND $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}+\mathrm{OH}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COO}^{-}+\mathrm{H}_{2} \mathrm{O}$

(ii)

Complete the sentence and explain why solution D does not act as a buffer when the other solution is added.

Solution D does not act as a buffer when is added to it. explanation

[ 1 ]

$\mathrm{H}_{2} \mathrm{SO}_{4}$ AND conjugate base of $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH}$ is not present

(e)

Manganese(II) hydroxide, $\mathrm{Mn}(\mathrm{OH})_{2}$ , is only slightly soluble in water.

The solubility of $\mathrm{Mn}(\mathrm{OH})_{2}$ in water is $3.28 \times 10^{-3} \mathrm{gdm}^{-3}$ at 298 K .

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(i)

Calculate the concentration of a saturated solution of $\mathrm{Mn}(\mathrm{OH})_{2}$ at 298 K.

\left[\mathrm{Mn}(\mathrm{OH})_{2}\right]=..............................\ \mathrm{mol\ dm}^{-3}

[ 1 ]

$3.69 \times 10^{-5}$

(ii)

Write an expression for the $K_{\mathrm{sp}}$ of $\mathrm{Mn}(\mathrm{OH})_{2}$ . Give the units of $K_{\mathrm{sp}}$ .

K_{s p}=

units =

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$\text { M1: } K_{\mathrm{sp}}=\left[\mathrm{Mn}^{2+}\right]\left[\mathrm{OH}^{-}\right]^{2}$

M2: units $=\mathrm{mol}^{3} \mathrm{dm}^{-9}$

(iii)

Use your answers to (d)(i) and (d)(ii) to calculate the value of $K_{sp}$ of $\mathrm{Mn}(\mathrm{OH})_{2}$ at 298 K.

K_{sp}=..............................

[ 1 ]

$2.0 \times 10^{-13}$

CAIE A-Level Chemistry 25.1 Acids & bases Question Bank

Exam points

Covered syllabus points

Covered syllabus points

25.1 Acids and bases question 1

25.1 Acids and bases question 2

Question (a)

Question (b)

Question (c)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question (d)

Question (i)

Question (ii)

Question (e)

Question (i)

Question (ii)

Question (iii)