(i) Any two of:

the two samples were collected at different times «as the lake continues to shrink, its remaining water becomes steadily more concentrated, so an earlier and a later sample would genuinely differ» ✔

OR

the samples were collected from different locations or depths in the lake ✔

OR

the two sources used different sampling or analytical methods/instruments ✔

(ii) mean $= \dfrac{31.60+27.96}{2} =$ 29.78 «$\mathrm{g\,dm^{-3}}$» ✔

«percentage difference $= \dfrac{31.60-27.96}{29.78}\times 100\% =$» 12.2 «%, accept 12-13%» ✔

(iii) Any three of:

prepare a series of standard solutions of a soluble salt of the ion, covering a range of known concentrations «e.g. by serial dilution of a stock solution» ✔

measure the absorbance of each standard solution with the spectrophotometer «at a fixed, suitable wavelength» ✔

plot a graph of absorbance (y-axis) against concentration (x-axis) ✔

draw a line/curve of best fit through the points, «this line is the calibration curve, later used to find the concentration of a sample of unknown concentration from its measured absorbance» ✔

(i) [1] each for two distinct, valid reasons; 'experimental error' or two versions of the same idea score once only. (ii) M1: correct mean of the two values (or equivalent working); M2: correct percentage, accept 12-13% for reasonable rounding; a calculation using Source 1 alone as the denominator (13.0%) is accepted as an alternative convention provided the working is shown. (iii) [1] for each of three distinct stages: preparing a range of standard solutions, measuring their absorbance, and plotting/drawing a best-fit line or curve.