Under SATP, magnesium was reacted with acid and 500 ml of hydrogen gas was collected over water.

A) what is the vapor pressure of water?
B) What is the pressure of the hydrogen gas?
C) What amount of hydrogen was produced?

I only know how to do the law of partial pressures and i don't think it applies to this? please help! thanks!

Of course it applies.

pTotal = pH2O + pH2
Look up the vapor pressure of H2O and substitute that for pH2O. I don't think everyone uses the same numbers for SATP but I will use P of 100 kPa or 0.987 atm so that is PTotal. If you looked up pH2O in mm then convert 0.987 to mm Hg also. That allows you to calculate pH2.

For C, I would use (p1v1/t1) = (p2v/t2)
p1 is pH2 from above in mm Hg.
p2 = 0.987*760 mm Hg.
t1 = 298
t2 = 298 and you really don't need to use t1 and t2 since they are the same.
v1 = 500 mL
v2 = ?

To answer these questions, you can make use of the ideal gas law, which states that PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the ideal gas constant, and T is the temperature in Kelvin.

A) To find the vapor pressure of water, you need to determine the total pressure inside the container in which the hydrogen gas is collected. Since the hydrogen gas is collected over water, you have to account for the partial pressure of water vapor.

To calculate the vapor pressure of water, you can use the Dalton's Law of Partial Pressures. According to this law, the total pressure is equal to the sum of the partial pressures of the individual gases present. In this case, the total pressure is the sum of the pressure of hydrogen gas and the vapor pressure of water.

Let's assume the pressure of hydrogen gas is P(H2) and the vapor pressure of water is P(water). Then:

Total pressure = P(H2) + P(water)

Since the total pressure is typically given in the problem statement or can be measured, subtract the given pressure of hydrogen gas from the total pressure to determine the vapor pressure of water.

B) The pressure of the hydrogen gas can be determined by subtracting the vapor pressure of water (which you found in part A) from the total pressure.

Pressure of hydrogen gas = Total pressure - Vapor pressure of water

C) To find the amount of hydrogen gas produced, you need to calculate the number of moles of hydrogen gas present in the collected sample.

First, you need to convert the volume of hydrogen gas collected over water to the volume of hydrogen gas collected at the dry conditions. To do this, use the fact that the vapor pressure of water at SATP is 23.8 mmHg.

Apply the following equation: P1V1 = P2V2, where the initial pressure and volume (P1, V1) refer to the conditions over water, and the final pressure and volume (P2, V2) refer to the dry conditions.

Once you have the dry volume of hydrogen gas, you can use the ideal gas law to calculate the moles of hydrogen gas.

n = PV/RT

Where n is the number of moles, P is the pressure (which you found in part B), V is the volume (which you calculated above), R is the ideal gas constant, and T is the temperature in Kelvin. Remember to convert the temperature to Kelvin if it is given in Celsius.

By following these steps, you should be able to find the answers to the given questions.