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CIE Jun 2015 v3 Paper 4 Q3
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![(a) The mass spectrum of the element magnesium is shown below. relative abundance (%) 50 23 24 25 m / e 26 27 (i) From the mass spectrum, complete the table with the relative abundances of the three isotopes. isotope relative abundance 24Mg 25Mg 26Mg [1] (ii) Use your values in (i) to calculate the relative atomic mass, Ar, of magnesium to two decimal places. Ar (Mg) =[1]UCLES 2015 9701/43/M/J/15<br />
(b) (i) Describe and explain the trend in the thermal stabilities of the nitrates of the Group II elements down the group.[3] When lithium nitrate, LiNO3, is heated, it readily decomposes giving off a brown gas. This reaction is similar to that which occurs when magnesium nitrate is heated, but it does not occur with other Group I nitrates. (ii) Suggest an equation for the action of heat on LiNO3.[1] (iii) Suggest why the Group I nitrates other than LiNO3 do not decompose in this way when heated.[1] [Total: 7]UCLES 2015 9701/43/M/J/15 [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v3_QP_-_Paper_4_CIE_Chemistry_A-level_3-0.jpg)
![Cambridge International A LevelMay/June 2015 Mark Scheme Syllabus Paper 9701 43 Page 3 (a) (i) 1 1 1 [Total: 7] 1 1 1 1 isotope relative abundance 24 Mg 25 Mg 26 Mg 7879 10 1211 (total must add up to 100 %) (ii) e.g. 0.78x24 + 0.10x25 + 0.12x26 = 24.34 (b) (i) nitrates become more stable (down the group) as the ionic radius increases or charge density on cation / ion decreases decreasing its ability to distort / polarise the NO3 / nitrate ion (ii) 4LiNO32Li2O + 4NO2 + O2 (iii) the charge density of the other cations are too small (to polarise the anion sufficiently so the anion is more stable) (a) (i) Ksp = [Ag 2 + (aq)] [SO4 3 (aq)] and units: mol dm 2 x (0.025) = 6.25 x 105 (ii) Ksp = (2 x 0.025) (b) Ag2SO4(s) latt sol 2- hyd or 2- (c) (i) Eo cell (= 0.800.77 =) (+)0.03V and Ag + / Ag or Ag / silver or right (ii) Ecell would be less positive / more negative + because the [Ag (aq)] (in the Ag electrode) is less than 1.0 mol dm (iii)no changeCambridge International Examinations 2015<br />](https://learnah.org/wp-content/uploads/2018/08/June_2015_v3_MS_-_Paper_4_CIE_Chemistry_A-level_3.jpg)