# What does the term 'by mass' mean with respect to different contexts?

I've seen the term 'by mass' being used in various sorts of contexts. Some, while explaining the law of definite proportions say, the 'same proportions by mass', some say 'constant chemical composition by mass'.

What does the term 'by mass' mean? Does it mean in a proportion/ratio, we have mass as both, the denominator and numerator?

When I say 'percentage by mass', what would that mean?

• You have the right idea, it's a ratio of masses. You might also see the phrase mass fraction Oct 21, 2020 at 14:43

In $$\ce{H2O}$$, the proportion of $$\ce{H}$$ counted by atoms is $$\ce{2/3 = 66.7}$$ %. In the same $$\ce{H2O}$$, the mass of $$\ce{2 H}$$ is $$2$$ u. As the mass of $$\ce{1 O}$$ is $$16$$ u, the proportion of H by mass in $$\ce{H2O}$$ is $$2/18 = 11.1$$%. This percentage $$11.1$$% is rather different from $$66.7$$%