Finding molarities of individual reagents given the chemical equation and total concentration in a container I am given a table that gives me the different total pressures at certain times. The decomposition of phosphine is the reaction: 

$$\ce{4 PH3(g) &lt;=&gt; P4(g) + 6 H2(g)}$$ 

The time/pressure values are:

\begin{array}{rr}
\mathrm{time\ [min]} &amp; \mathrm{pressure\ [Torr]} \\
\hline
0 &amp; 100 \\
40 &amp; 150\\
80 &amp; 167\\
100 &amp; 172\\
\end{array}

This reaction takes place at 950K. I can then find the molarity of the total gas mixture at each time using $$\frac{n}{V}=\frac{P}{RT}$$

I need to know how to get the pressure of the $\ce{P4}$ for each individual time. How would one go about doing this?
The assumption you need to make is that at time zero, only $\ce{PH3}$ is present.  So relative to the moles at time zero, or $N_0$, you should find $\Delta N = N(t) - N_0$, the total new moles formed by the reaction since it began.

The reaction converts 4 moles of gas to 7.  

So pressure increases mean the reaction is moving to the right.  Every mole of pressure increase must be $\frac{6}{7}$ due to $\ce{H_2}$ and $\frac{1}{7}$ due to $\ce{P4}$ formation.

So just find the total new moles of gas and multiply by $\frac{1}{7}$.