[student8607]

Hard water contains alkaline earth cations such as Ca²⁺ which reacts with CO₃^2- to form insoluble deposits of CaCO₃. Will a precipitate of CaCO₃ form if a 250mL sample of hard water having [Ca²⁺] = 8.0x10^-4M is treated with the following
a) 0.1mL of a 2.0x10^-3M Na₂CO₃
b)10mg of solid Na₂CO₃

I'm fine once I get everything set up. First, I have to determine initial moles from the given molarities.

What are the initial volumes of step a. Is 250mL a total? So CO₃ is 0.1 and Ca is 24.9? OR is it 250mL of Ca and 0.1mL for CO₃

Again, what volumes should I use for part B?

[Borek]

Assume 250 mL is a total. Even if it is not, 250.1 and 250.0 differ by 0.04%, after rounding your final result you will be not able to tell what volume was used.

[student8607]

OK.

So for a. 249.9mL of Ca & 0.1mL of CO3

How about for b? It doesn't give a volume?

[AWK]

I'm fine once I get everything set up. First, I have to determine initial moles from the given molarities.

Why for?
You need molarities for the equilibrium calculations

Borek pointed out you can use safely volume 250 ml instead of 250,1 ml (error 0,4 %)

b)10mg of solid Na2CO3

convert mass to moles, then to concentration. Again assume volume of 250 ml.

[student8607]

This is the method I was taught but perhaps I don't need to use it here.

1. Take initial molarities and times it my volumes present to get initial moles of each reactant
2. take initial moles of each reactant and multiply them by the new/combined volume of the solution
3. use these molarities in Q equation to compare with Ksp