Simple Chemistry Question, Please Help!? I thought this would be as easy as adding, but I keep getting the wrong answer

Question

A closed-end manometer was attached to a vessel containing argon. The difference in the mercury levels in the two arms of the manometer was 12.6 cm. Atmospheric pressure was 783 mm Hg. The pressure of the argon in the container was ____ mm Hg.
126
770
661
882
796

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Answer 1 · 2017-11-16T22:12:07

Simple is a relative term!

We are looking to compare the atmospheric pressure to pressure exerted by the vapor of the liquid in the bulb on the right.

www.webassign.net

The liquid in the "U" is mercury. Hence, $m m H g$ $mm Hg$ . Let's standardize units first, then solve the problem.

$12.6 c m \cdot \frac{10 m m}{c m} = 126 m m$ $12.6cm * (10mm)/(cm) = 126mm$

Assuming that argon's vapor's pressure is less than atmospheric:

$783 m m - 126 m m = 657 m m$ $783mm - 126mm = 657mm$

Assuming that argon's vapor's pressure is greater than atmospheric:

$783 + 126 m m = 909 m m$ $783 + 126mm = 909mm$

It looks like our first assumption was right, the answer may be (C). Someone correct me if I'm wrong!