Tutoring high school chemistry, this topic is perennial.  The tutor embarks it gently on this warm, late-summer night….

Most people are familiar with the fact that water is H₂0: a water molecule is made from two hydrogen atoms combined with one oxygen atom. However, perhaps not as many people realize that water is only 11% hydrogen.

The reason that water is only 11% hydrogen, even though it has two hydrogen atoms but only one oxygen: percent composition is calculated from mass rather than number of atoms. Looking at the periodic table, we see that hydrogen (H) has a mass of 1.0 (the mass is the decimal number); oxygen (O) has mass 16.0 (rounded to one decimal place). Percent composition is given by the following:

%comp=(mass of specific element)/total mass

To get water’s total mass, add the masses of the two hydrogen atoms plus the one oxygen atom: 1.0+1.0+16.0=18.0. In water, the percent composition of hydrogen is

%comp hydrogen=(mass H present)/(total mass H₂O)=(1.0+1.0)/18.0=11%

The percent composition of oxygen in water is

(mass O present)/(total mass H₂O)=16/18=89%

Of course, the percent hydrogen and the percent oxygen must add to 100%, since they are the only two elements present. It’s comforting, therefore, that 11% + 89% = 100%:)

When calculating percent composition, chemists use mass rather than number of atoms. Some measures of concentration are determined from a similar perspective. I’ll pursue this idea in future posts:)

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.