# A sample of oxygen gas occupies a volume of 250 mL at 740 mmHg. What volume will it occupy at 800 mmHg?

${P}_{1} {V}_{1} = {P}_{2} {V}_{2}$
I don't like this question very much, because it suggests that you can get a column of mercury that is $800 \cdot m m$ high. Try this, and you'll get mercury all over your laboratory, where it will inhabit every crack, and every cranny. Cleaners also will not touch your lab until you clean it up.
One atmosphere will support a column of mercury $760$ $m m$ high, and this is as high as you go. Would you care to use new units of pressure? Your teacher should know this.