Need help with 2 AP chem questions.

Question

Q1: A piece of magnesium was added to excess HCL at 725 mm Hg and 28.0°C. If 38.0 mL of hydrogen was collected over 20.0°C water, what is the mass of the magnesium?

Q2: In the reaction Mg(s) + 2HCL(aq)--> MgCl2(aq) + H2(g), a student obtained a sample of magnesium and reacted it with excess HCL. 29.9 mL of H2 was collected under the following conditions: Room temp= 20.0°C, pressure= 775 mm Hg, How many grams of Mg were consumed?

Answer 1

1. Write the balanced eq:

Mg + 2HCl ==> MgCl₂ + H₂

moles H₂ formed: PV = nRT. P = pressure AFTER subtracting vapor pressure of water at 20 degrees from 725 mm. Tge v.p. Of water can be found in a text book or reference table. Solve for n=PV/RT

Next, moles Mg = moles H₂ based on stoichiometry of the equation.

Finally, mass Mg = moles Mg x atomic mass Mg

2. Same approach as in #1. Find moles of H₂ after subtracting vapor pressure of water and solving for n in n = PV/RT. Then, moles Mg consumed = moles H₂ formed. Mass of Mg consumed = moles Mg x atomic mass of Mg