Mg(s) + 2 H+(aq) → Mg2+(aq) + H2(g) A student performs an experiment to determine the volume of hydr — Gases Chemistry Question
Problem Context
Mg(s) + 2 H+(aq) → Mg2+(aq) + H2(g)
A student performs an experiment to determine the volume of hydrogen gas produced when a given mass of magnesium reacts with excess HCl(aq), as represented by the net ionic equation above. The student begins with a 0.0360 g sample of pure magnesium and a solution of 2.0 M HCl(aq).
Model Answer
1 mol Mg / 24.30 g Mg × 0.0360 g = 0.00148 mol Mg
One point is earned for the correct number of moles of Mg.
Note: For errors in significant figures or mathematics, there is a 1 point deduction; For the entire question, only 1 point total can be deducted for each type of error.
Model Answer
0.00148 mol Mg × (2 mol H+ / 1 mol Mg) = 0.00296 mol HCl
One point is earned for the correct number of moles of HCl.
Model Answer
P = 749 torr − 21 torr = 728 torr
One point is earned for the correct pressure.
Model Answer
PV = nRT ⇒ V = nRT / P
728 torr × (1 atm / 760 torr) = 0.958 atm
V = (0.00148 mol H2)(0.0821 L atm mol^-1 K^-1)(296 K) / 0.958 atm = 0.0375 L
One point is earned for the correct moles of H2(g).
One point is earned for agreement of units of P (from part (c)) and R.
One point is earned for the correct temperature substitution and calculated volume.
Model Answer
2.0 mol HCl / 1000. mL × 50.0 mL = 0.10 mol HCl
1000. mL / 12.3 mol HCl × 0.10 mol HCl = 8.1 mL
Pour some distilled water into the volumetric flask so it is about half full. Use the graduated cylinder to measure 8.1 mL of 12.3 M HCl and pour it into the volumetric flask. Add a few mL of distilled water to the graduated cylinder, swirl, and add the liquid to the volumetric flask. Swirl the flask to mix. Carefully add more distilled water to the flask, using the dropper for the last few mL to fill the flask to the mark with distilled water.
One point is earned for calculating the correct volume of 12.3 M HCl.
One point is earned for adding acid to water in the dilution process.
One point is earned for filling the volumetric to the 50.00 mL mark with distilled water.