Measuring mole fraction in lab using Raoult's law
The mole fractions in the gas phase also depend on the relative volatility of the two liquids, so they will not in general be equal to the mole fractions in the liquid mixture. For instance, if you have components A and B, then their vapor pressures will be 

$$ \begin{align} p_A &amp;= \chi_A p^\circ _A \\ p_B &amp;= \chi_B p^\circ _B \end{align}$$

where $\chi_i$ is the mole fraction of component i in the liquid and $p^\circ_i$ is the vapor pressure of the component when pure. According to Dalton's law the mole fraction of a given component in the ideal gas is proportional to its contribution to the total pressure so that 

$$\begin{align}  p&amp;=p_A+p_B \tag{1}\\ &amp;= \chi_A p^\circ _A + \chi_B p^\circ _B \\ &amp;= \chi_A (p^\circ _A -p^\circ _B) + p^\circ _B \end{align}$$

From this last expression you can conclude that the total pressure above the mixture will vary linearly with the mole fraction of either component in the liquid. 

You can also show that the mole fraction of either component in the liquid is not equal to that in the gas. For instance for component A you can write by combining Raoult's and Dalton's laws that

$$\chi_{A,g}=\frac{p_A}{p}=\chi_{A,l}\frac{p_A^\circ}{p} \tag{2}$$

where the mole fractions in the liquid and gas are labelled explicitly with subscripts $l$ and $g$. It should be clear that $\chi_{A,g} \neq \chi_{A,l}$ unless $p=p_A^\circ$, but the latter condition is only satisfied for the pure substance A.