Q.) A student performs this lab using 0.122 grams of magnesium with excess acid. She collects 127 mL of product gas (over a water bath) at 23.0°C and an atmospheric pressure of 757.2 mmHg.

A.) Using the mass of magnesium, calculate the number of moles of product gas produced, via stoichiometry.

B.)The gas collected is actually mixed with a small amount of water vapor, because it is collected over a water bath. The total of these two pressures is equal to atmospheric pressure. That is

P(atm) = P(gas) + P(water vapor)

Based on the data above and the table of water vapor pressure in the lab procedure, what is P(gas)?


C.) Using these values of P(gas), V(gas), n(gas) and T(gas), calculate the student's experimental value of R (the gas constant).

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For the first part, find the volume of gas you expect to obtain in the metal + acid reaction, using the balanced equation for the reaction. I will use the ionic equation, since you didn't mention what was the acid used. The results will be the same though.



So, from this, you can find the number of moles of Mg present, then the number of moles of gas expected.

Now, find the P(gas). You have the total pressure, that is 757.2 mmHg, and the pressure of water vapour from your table.

Then, for the last part, find the volume occupied by the gas. The total volume is 127 mL. Remove the volume occupied by water vapour, and you get the volume occupied by the gas.

Now that you have all the data, you should know the formula:



To find the value of R.

I hope it helped! :)