What is the volum required to precipitate?

Question

what is the volume in mL of 0.25M AgNO3 required to precipitate all the chromate ion from 20. mL of a solution containing 100.g of Na2CrO4 per liter?

2Ag ＋ CrO4 2- → Ag2CrO4(s)

Answer 1

find moles of Na2CrO4

100 g @ 161.97 g/mol = 0.6174 moles of Na2CrO4 are in one litre of solution

find moles Na2CrO4 in the 20 ml"s:

20 ml @ 0.6174 moles / 1000 ml = 0.01235 moles CrO4)-2

since it takes 2 Ag+'s to make 1 Ag2CrO4:

0.01235 moles of Na2CrO4 @ 2 moles Ag+ / 1 mol CrO4)-2 = 0.02470 mol Ag+ is needed

find the volume you requested:

0.02470 moles @ 0.25 mol/litre = 0.09878 litres

which is 98.78 millilitres of AgNO3

your answer (3 sig figs) : 98.8 ml of AgNO3

youir answer (2 sigfigs) : 99 ml of AgNO3

"20.ml" &" 0.25M" only appear to have 2 sigfigs