What is Composition variables?

For a constituent $i$ of a phase containing the amounts $n_j$ of all its constituents:

• its mole fraction is $x_i = n_i/\sum_j n_j$, and its mass fraction $w_i = m_i/\sum_j m_j$; both lie between 0 and 1 and sum to 1;
• in a solution of volume $V$, its molar concentration is $c_i = [i] = n_i/V$, in $\mathrm{mol}/\mathrm{L}$;
• in a gas mixture at total pressure $p$, its partial pressure is $p_i = x_i\,p$.