Question

A sample consists of one or more of the following salts: Cu(CH3CO2)2 (acetate), BaCl2, (NH4)2SO4, SrCO3, KNO3. The sample, when treated with water, gives a white precipitate that is filtered off and a blue solution. The precipitate is a white solid that is completely insoluble, both in water and in 6M HCl solution. Which compounds MUST be present? Explain why. Write the formulae and present your evidence. 4 Which compounds MUST be absent? Explain why. Write the formulae and present your evidence. Which compounds are INDETERMINATE - unable to determine if present or absent. Write the formulae.

Answer 1

Cu(CH3CO2)2 (acetate), BaCl2, (NH4)2SO4, SrCO3, KNO3

when in water, they form white precipitate

possible precipitates --> BaSO4, BaCO3 only, both are white, therefore , we need furhter proof

note that Cu+2 + SO4-2 will be soluble

CuCO3 will not be formed, since this is not white

Cu2+ ions are presnet, since "blue color solution"

b)

when adding HCl, it becomes soluble

BaSO4 --> not likely to dissolve in acid

BaCO3 + HCl --> H2CO3- + Ba+2

likely to dissolve

we now know BaCO3(s) is present

then

a)

MUST

Cu(CH3CO2)2 due to Cu2+ color

BaCl2; due to BaCO3 precipitation

SrCO3; due to BaCO3 preciitaiton

b)

not sure --> KNO3, since no test

c)

for sure not present is

(NH4)2SO4

since HCl addition should give off NH3(g) bubbles