A sample of nitrogen gas is collected overwater at a temperature of 23◦C. What is the pressure of the nitrogen gas if the atmospheric pressure is 744 mmHg?

Question

Answer in units of mmHg.

Not sure of formula for this

Answer 1

A gas 'collected over water' is a mixture of water vapor and the gas. So
P,total = P,gas + P,water vapor

You'll solve for P,gas and to do that, you'll need data for vapor pressure of water at 23 C.
P,water vapor = 21.1 mm Hg (different sources may show different values)

Substitute:
744 = P,gas + 21.1
P,gas = ?

Answer 2

I guess I am dumb I don't get it

Answer 3

What don't you get? Dalton's law of partial pressures tells us that the total pressure of a system is the sum of the partial pressures of each gas. You have two gases in this system; i.e., N2 gas and water (as a vapor which is a gas). So according to Dalton's Law, the total pressure, which is given in the problem as 744 mm, is the total of the two pressures.
Ptotal = pH2O + pN2
Ptotal = 744 (from the problem)
pH2O = 21.1 (from a table)
pN2 = ?