[jennielynn_1980]

silicon oil (density = 1.30g/cm^3) is often used as the liquid in a manometer.  A silicon oil manometer is operated at 25.4 C.  One side is connected with a gas bulb, with the other side open to the atmosphere.  If the level of oil in the side attached to the bulb is 6.24 cm higher than the side open to the atmosphere, what is the pressure of the gas in the bulb if the atmospheric pressure is 740.5mmHg?

This question is really getting me!  So I know that the height of the oil is related to the height of the liquid and density of the liquid.  I think that the pressure inside the gas bulb is less than the atmospheric pressure because the height of the liquid column is lower on the side open to the atmosphere so the atmosphere must be exerting more pressure than the liquid.  I just don't even know where to start.  I have tried P=g x h x d and that isn't the correct answer.  I am assuming I am missing something to do with the density of oil being different to that of Hg.  Could someone tell me if I am on the right track? 
Thanks so much!

[Borek]

I have tried P=g x h x d and that isn't the correct answer.  I am assuming I am missing something to do with the density of oil being different to that of Hg.

Equation is the right one, show what you did.

[jennielynn_1980]

P_bulb = 9.81 m s^-2 x 1.30 g cm³ x 6.24 cm
        = 79.579 g cm^-2 sec^-2

So now I am not sure what my units are for one and whether I should be converting cm to m before the calculation.  I think the answer is in Pascals because 1 Pa = 1 N m^-2 and 1 N is 1 kg m s^-2

[Borek]

That's more or less what I expected - you messed up units.

First of all - you can't multiply m by cm and state your result is in cm².

But that's only part of the problem, as your units are wrong, probably because of some error in your math.

Many ways of skinning that cat. Start converting everything to the same length units - either cm or m. Looks like cm will be easier.

There is a trick that can be used to make calculations easy and fast, but tricks are good when you already mastered the systematic method.

[jennielynn_1980]

Thanks so much for your help Borek.  I switched my 9.81 units to cm and then finally got the right answer!