Conservation of Mass The easiest principle to appreciate is conservation of mass. Except for nuclear reactions, an element’s total mass at the end of a reaction must be the same as that present at the beginning of the reaction; thus, an element serves as the most fundamental reaction unit. Consider, for example, the combustion of butane to produce CO2 and H2O, for which the unbalanced reaction is C4H10(g) + O2(g) ® CO2(g) + H2O(g) All the carbon in CO2 comes from the butane, thus we can select carbon as a reaction unit. Since there are four carbon atoms in butane, and one carbon atom in CO2, we write 4 ´moles C4H10 = 1 ´ moles CO2 Hydrogen also can be selected as a reaction unit since all the hydrogen in butane ends up in the H2O produced during combustion. Thus, we can write 10 ´ moles C4H10 = 2 ´ moles H2O Although the mass of oxygen is conserved during the reaction, we cannot apply equation 2.3 because the O2 used during combustion does not end up in a single product. Conservation of mass also can, with care, be applied to groups of atoms. For example, the ammonium ion, NH4+, can be precipitated as Fe(NH4)2(SO4)2 × 6H2O. Selecting NH4+ as the reaction unit gives 2 ´moles Fe(NH4)2(SO4)2 · 6H2O = 1 ´ moles NH4