Note that the physical states of reactant and product materials as well as the concentration of the
aqueous sulfuric acid solution are described in the equation. This level of specificity is required in
that the value of rH depends markedly on these details. For example, rH is significantly affected
by the sulfuric acid concentration and also depends on the identity of the solvent, water. In
equation (R), the sulfuric acid concentration is expressed in terms of molal concentration units,
defined as moles of solute per kg of solvent. The symbol (aq, 1m) denotes a solution containing 1
mol of H2SO4 (FW = 98.0 g/mol) in 1000 g of the solvent water. The molal concentration unit is
particularly useful for solutions prepared by weight. Thus, a 1m H2SO4 solution is readily prepared
by adding 98.0 g of pure H2SO4 to 1000 g of water. Once prepared, the solution is conveniently
measured out by weight. A 1098.0 g portion of 1m H2SO4 contains just 1 mol of H2SO4 and 1000 g
of water; a 109.8 g portion contains 0.1 mol H2SO4 and 100 g H2O, etc.