Question

1. How many grams of solution are needed to obtain 40g K2CO3 from a 10% (aq) solution?

2. How many grams of chlorine are present in a 1L of a 650 PPM (aq) solution?

Answer 1

mass of 1 mole of K2CO3=2*39+12+3*16=78+12+48=138
mass of 1 mole of water(H2O)=2*1+16=18
i don't remember more than that.
i have no reference book with me.
i have read about 40 years back.

Answer 2

no need for moles here.

The 10% is probably by mass (though this should really be specified).

This means that in 100 g of solution, 10 of those are potassium carbonate.

if you need 40 g, then you need 400g of solution.

for number two use the fact that 1 ppm = 1 mg/mL, thus the density is 650 mg/mL.