Question

helium gas, 3x10-4g, is dissolved in 200g of solution. Express this concentration in parts per million.

Answer 1

(3x10^-4)/200 = 1/5x10^-6 = 1.5 ppm

Answer 2

3x10-4g, is dissolved in 200g of solution. Express this concentration in parts per million.
The easy way is to show it in percent; i.e. in parts per hundred so that is
3E-4g/200g = 1.5E4g/100g then multiply numerator and denominator by 10^4 or
1.5E-4g.......10,000
---------- X ----------- = 1.5 ppm
100g............10.000
The spacing won't look quite right but you get the idea.
Another way is to remember that ppm is mg/1000 g or
3E-4g/200 g = 0.3 mg/200 g
0.3 mg/200 x 5/5 = 1.5 mg/1000 g = 1.5 ppm

Answer 3

Note that oobleck made a typo. That 1/5 he meant to be 1.5.