Chemistry Precipitate question?

Question

Starting with 11.8 ml of 3.40 × 10−2 M AgNO3, how many grams of KBr can be added to the solution before AgBr begins to precipitate ?

Answer 1

According to http://www.ktf-split.hr/periodni/en/abc/kpt.html
AgBr has Ksp=5.35*10^-13

AgNO3 dissociates 100% with a stoichiometry 1 AgNO3 : 1 Ag+ , thus [Ag+]= 0.0340 M
The concentration of Br- for a saturated solution of AgBr can be calculated from the Ksp

Ksp= [Ag+][Br-] =>
[Br-]= Ksp/[Ag+] = (5.35*10^-13)/0.034 = 1.57 *10^-11 mole/L
This is the maximum concentration of Br- you can have in the solution without forming a precipitate.Let's find out to how many grams KBr that corresponds to.

KBr is a strong electrolyte that dissociated with a stoichiometry 1 KBr : 1 Br-, thus 1.57*10^-11 mole/L Br- come from 1.57*10^-11 mole/L KBr
We have 0.0118 L, thus 0.0118*1.57*10^-11 = 1.853*10^-13 mole KBr
But mole= mass/MW =>
mass =mole*MW = (1.853*10^-13) *118.9 =2.20*10^-11 g KBr