Edexcel A-Level Chemistry A2 14.14 Ka for a Weak Acid From Experimental Data Given the Ph of a Solution Containing a Known Mass of Acid Questions

calculation of concentration of acid (1)

- calculation of $\left[\mathrm{H}^{+}\right]$value

(1)

- expression or substitution into expression

(1)

- evaluation of $K_{a}$

(1)

Example of a calculation:
$0.0150 \div 94.5=0.0001587 / 1.59 \times 10^{-4}$$0.0001587 \div 1.5=0.00010582 / \mathbf{1 . 0 5 8} \times \mathbf{1 0}^{-\mathbf{4}}\left(\mathrm{mol} \mathrm{dm}^{-\mathbf{3}}\right)$$\left[\mathrm{H}^{+}\right]=10^{-3.42}=\mathbf{3 . 8 0 2} \times \mathbf{1 0}^{-\mathbf{4}}\left(\mathrm{mol} \mathrm{dm}^{-3}\right)$
( $\left[\mathrm{H}^{+}\right]^{2}=1.445 \times 10^{-7}$ )
$K_{\mathrm{a}}=\frac{\left[\mathrm{H}^{+}\right]^{2}}{[\text { acid }]}$ or $K_{\mathrm{a}}=\frac{\left[1.445 \times 10^{-7}\right]}{\left[1.058 \times 10^{-4}\right]}$$K_{\mathrm{a}}=0.0013655 / 1.37 \times 10^{-3}\left(\mathrm{~mol} \mathrm{dm}^{-3}\right)$
TE from M3 if answer < 1
Allow $1.36 \times 10^{-3}\left(\mathrm{~mol} \mathrm{dm}^{-3}\right)$ for 4 marks
Ignore SF except 1 SF
Correct answer with no working scores 4
Penalise incorrect units for M4

(4)

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