$M_{tot}=\dfrac{m_{tot}}{n_{tot}}=\sum_{i=1}^ky_iM_i\quad\text{or}\quad M_{tot}=\left[\sum_{i=1}^kc_iM_i\right]^{−1}$ The [[Mass|mass]] and [[Mole|mole]] fractions are therefore related by the following: $c_j=y_j\dfrac{M_j}{M_{tot}}=\dfrac{y_jM_j}{\sum_{i=1}^ky_iM_i}$ $y_j=\dfrac{\dfrac{c_j}{M_j}}{\sum_{i=1}^k\dfrac{c_i}{M_i}}$