Parts per million problem?

Question

if 1,000 g of water from Luboc River contains 0.0075 g of Ca2+ ion, how many oom by mass of Ca2+ is present in Luboc River?

Answer 1

Just do it in steps or set up a ratio.

Ratio method:

0.0075 / 1000   =   x / 1000000             x = 7.5 ppm

Note, we really should use the total mass (1000.0075), but we assume that this is approximately 1000 for the number of sig figs we have.  Try it yourself and you will see it is a good approximation.

Step method:

0.0075/1000 = 0.075/10000 = 0.75/100000 = 7.5/1000000 so 7.5

You will see people say 1ppm = 1 mg/L.  Note in the back of your brain, this only applies for diltute solutions and is an approximation.  If you have a 30% iron solution 1ppm does NOT equal 1mg/L.

Answer 2

ppm (in water) is the same as mg/l so 0.0075 g is 7.5 mg and 1000 g

(of water) is 1 l so it's 7.5 ppm Ca+2.    And BTW 1% is 10,000 ppm.