2 replies to this topic

[JH226]

Hi,
Just wondering if any one could help me with this chemistry question:

A mixture of calcium chloride and calcium sulphate is known to contain 0.6 moles of chloride ions and 0.2 moles of sulphate ions. How many moles are present?

Please help, Thanks

[Marcus]

well obviously the total number of moles is 0.6 mol Cl^- + 0.2 mol SO₄^2- + moles of calcium
The key part of this question is working out the moles of calcium... since calcium is 2+, the formulae must be CaCl2 and CaSO4, therefore the total moles of calcium is 0.5* moles Cl + moles SO4
ie total moles is 0.6+0.2+0.3+0.2=1.3 moles

[donaldc50]

Hi. Just read this one. I am useless at keeping an eye on this forum. Anyway. The question was not really complete. I think the real question would relate to the number of moles of calcium ions only. If so, 0.6 moles chloride would mean there was 0.3 moles calcium from this source as there is a 2:1 ratio. Also if 0.2 moles sulphate there would be 0.2 moles calcium from this source as 1:1 ratio. Therefore total calcium ions is 0.3 + 0.2 = 0.5 moles. it is most common in this type of multi-choice question two sorces of one ion are given and you need to consider the formula of each compound to find the relevant ratios to make sense of the question. Similar questions mix compounds or volumes and ask for concentrations and so total final volume needs to be taken into account. eg 0·5 mol of copper(II) chloride and 0·5 mol of copper(II) sulphate are dissolved together in water and made up to 500 cm3 of solution. What is the concentration of Cu2+(aq) ions in the solution in mol l–1? There is a total of 1 mole copper(II) ions in 0.5 litre. So use c = n / v (l) =
2 mol/l. So always look for compound formulae and volumes before calculating. Beware of volumes not 1 litre. Hope this helped.