Question

A 3.2-gram sample of air contains 0.000 74 gram of hydrogen cyanide. Determine the concentration, in parts per million, of the hydrogen cyanide in this sample.

Answer 1

 The answer is: 231.25 ppm. 
To solve this, compute first the percentage of hydrogen in the 3.2 g air sample. % = (0.00074g/3.2g)*100 = 0.023125%
1% = 10,000ppm <--- use this as conversion factor. 

0.023125%(10,000ppm/1%) = 231.25 ppm