I did this experiment where I filled a graduated cylinder with water. The I put a stopper on it and put it under water and removed the stopper. The with a modified lighter (that wouldn't release sparks) I added butane gas. I got the following results:

mass of lighter = 15.15 g
final mass of lighter = 15.00 g
volume of butane = 80.0 mL
temperature = 25.0 degrees celsius
atmosperic pressure = 101.4 Kpa

How would I find the water vapour pressure?

Thank you.

see above.

To find the water vapor pressure, you can use the ideal gas law equation:

PV = nRT

Where:
P = pressure
V = volume
n = number of moles
R = ideal gas constant (8.314 J/(mol·K))
T = temperature in Kelvin

First, convert the temperature from degrees Celsius to Kelvin by adding 273.15:

T = 25.0 + 273.15 = 298.15 K

Next, let's calculate the number of moles of the butane gas using its volume and the ideal gas law. Assuming ideal gas behavior, you can use the equation:

PV = nRT

Rearrange the equation to solve for n:

n = PV / RT

Substitute the given values into the equation:

n = (101.4 kPa) * (80.0 mL) / ((8.314 J/(mol·K)) * (298.15 K))

Make sure all the units are consistent. Convert mL to L by dividing by 1000 and convert kPa to Pa by multiplying by 1000. The resulting units will be in moles.

n = (101.4 * 10^3 Pa) * (80.0 * 10^-6 L) / ((8.314 J/(mol·K)) * (298.15 K))

Calculate n using this equation.

Now, consider that the butane introduced into the graduated cylinder will occupy the remaining space above the water level. Therefore, the total pressure inside the graduated cylinder is equal to the atmospheric pressure plus the vapor pressure of water:

P_total = P_atm + P_water vapor

Since we know the atmospheric pressure is 101.4 kPa, we just need to find the water vapor pressure contribution.

Using Dalton's law of partial pressures, we can calculate the water vapor pressure. Dalton's law states that the total pressure of a mixture of gases is the sum of the partial pressures of each gas.

P_water vapor = P_total - P_atm

Referencing the given data, we know P_total is equal to the pressure inside the graduated cylinder, which is equal to the pressure at the top of the water column. So, P_total is equal to the pressure of the butane gas in the graduated cylinder.

P_total = pressure of butane gas

Calculating P_water vapor:

P_water vapor = P_total - P_atm

Substituting the values:

P_water vapor = (mass of lighter - final mass of lighter) / n

P_water vapor = (15.15 g - 15.00 g) / n

Finally, substitute the value of n that you calculated earlier:

P_water vapor = (15.15 g - 15.00 g) / (value of n from previous calculation)

Solve the equation to find the water vapor pressure.