Most people are familiar with the fact that water is H2<\/sub>0: 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.<\/p>\n 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:<\/p>\n %comp=(mass of specific element)\/total mass<\/p>\n 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<\/p>\n %comp hydrogen=(mass H present)\/(total mass H2<\/sub>O)=(1.0+1.0)\/18.0=11%<\/p>\n The percent composition of oxygen in water is<\/p>\n (mass O present)\/(total mass H2<\/sub>O)=16\/18=89%<\/p>\n 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%:)<\/p>\n 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:)<\/p>\n