Question 3b7de

Oct 28, 2016

Show all your units, and write subscripts clearly. $P V = P V$ doesn't mean anything, because it's just saying $1 = 1$, and that's obvious.

${P}_{1} {V}_{1} = {P}_{2} {V}_{2}$,

because temperature and mols were held constant. Furthermore, you made yourself write an extra step. For convenience, you can always calculate the final pressure in the units of the initial pressure.

So here's the short way to do it:

${P}_{2} = \frac{{P}_{1} {V}_{1}}{{V}_{2}}$

= (("755 mm Hg")("2.7 L"))/("4.2 L")#

$=$ $\textcolor{b l u e}{\text{485 mm Hg}}$ $\implies$ $\text{490 mm Hg}$ for sig figs.

(And clearly, $485 \ne 49$.)