Question

What is the concentration of a solution in parts per million if 20.0 grams of Na2S is dissolved in 4.00 x 10^5 grams of water?

Answer 1

`"Concentration"-=50*"ppm"....`

Explanation:

`1*"ppm"=1*mg*L^-1`...why? Because in each litre of water (and `"ppm"` solutions are invariably quoted for aqueous solutions), there are `1000*gxx1000*mg*g^-1-=1000000*mg` (considerations of density do not really matter here).

And so we take the quotient....

`"mass of solute in milligrams"/"volume of solution in litres"`

`(20.0*gxx10^3*mg*g^-1)/(4.00xx10^5*gxx10^-3*g*L^-1)` `-=` `50*mg*L^-1=50*"ppm"`...

And please note that this concentration refers to the concentration of sodium sulfide...i.e. there are `50*mg` of this salt dissolved in `1000000*mg-=1000*g-=1*L` of water.....