could someone please explain how they came up with the answer? Thanks
What this question is testing is your knowledge on the basic and acidic nature of salts, as well as the equilibrium properties.
To increase the concentration of the (CrO4)2-, you must remove the hydrogen ions to shift the equilibrium left.
For A sodium nitrate, the sodium comes from sodium hydroxide, a strong alkali so it will remain ionised. The nitrate comes from nitric acid, a strong acid and will also remain ionised. Therefore sodium nitrate will have no effect.
For B sodium chloride, similar to A as sodium comes from a strong alkali and the chloride comes from hydrochloric acid, a strong acid so it will also remain ionised. Therefore this compound will also have no affect.
For C sodium acetate, the sodium will remain ionised, however the acetate comes from acetic acid (ethanoic acid) this is a weak acid and therefore some of the hydrogen ions will form with the acetate ions, thus reducing the concentration of the H+ and giving the desired result.
For D ammonium chloride, the chloride will remain ionised, although the ammonium comes from a weak alkali ammonium hydroxide, therefore it will form with the hydroxide ions present (note the hydroxide ions are produced from the water on the right hand side) therefore the equilibrium will shift to the right, which is the wrong direction as they want (CrO4)2-
