Question

what do you mean by parts per million?

Answer 1

1 gram something in 1 million grams total which is the same as 1 g something + 999,999 g something else.
If we are talking about 1 ppm salt in water, we have
1 g salt in 1,000,000 g salt solution which really means 1 g salt + 999,999 g water.
Or you can do it 1 mg/1 million mg
1 pound/1 million pounds etc.
If you are to convert something to ppm, I like to use the definition of 1 ppm = 1 mg/L. That is, 1 mg NaCl/L soln is 1 ppm.
ppm stands for parts per million.
You may also see ppt (parts per thousand) or ppb (parts per billion)
1 ppb = 1 microgram/L
You are aware, I'm sure, that pph (parts per hundred) is the same as percent.