The relation connecting the fugacities of various components in a solution with one another and to composition at constant temperature and pressure is called the __________ equation.

The gibbs duhem equation is an equation that relates the partial modal properties of different species in the solution with composition at constant temperature and pressure and the equation is given by :
Writing the gibbs duhem equation in terms of partial gibbs free energy :

$$\sum\nolimits_{i\, = \,1}^n {} {x_i}d{\overrightarrow G _i} = 0$$

Since $$d\stackrel{\rightharpoonup }{G_l}=RTdln\left({\stackrel{⌢}{f}}_i\right)$$

We can write \[\sum\nolimits_{i\,=\,1}^{n}{{}}{{x}_{i}}d{{\left( ln\overset\frown{f} \right)}_{i}}=0\]
So, by gibbs duhem equation the composition and fugacity are related.