Question

please help. im desperate. my teacher is not that great and i just need an example of finding pressure.
***The density of mercury is 13.5 g/mL and the density of water is 1.00 g/mL. If a mercury barometer reads 727 mmHg, what is the barometric pressure in cmH2O?***

Answer 1

If a barometer reads 727 mmHg that means that the pressure in the atmosphere is able to support a height of 727 millimeters of mercury. Normally this is inside of a container (tube) with an arbitrary area that I'll call A.

So P = F/A where F = mg = density * volume * g = density * Area * height * g.

This means that P = density * height * g (because areas cancel out)

So now you have an equation for Pressure in terms of height and density (which are the only pieces of information you're given)

Since the pressure acting on the barometer is the same (regardless of whether you have mercury or water in it), we can set the pressure felt by the column of water to the pressure felt by the column of mercury.

P_water = P_mercury
density_water * height_water * g = density_mercury * height_mercury * g
..... g cancels out so

density_water * height_water = density_mercury * height_mercury. Just plug in your values at this point and solve for height_water.

Answer 2

Thank you! this helps alot!