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May 2, 2016

# Homework Help: chem 3A

Posted by shylo on Friday, July 25, 2008 at 8:40pm.

please help me with these homework problems from chemistry because the teacher did not give enough information to some of these chemistry work problems.

1) 50.0 g of aluminum is reacted with excess sulfuric acid. The chemical equation for the reaction is:
2Al + 3H2SO4 --> Al2(SO4)3 + 3H2
If 58.7 L of H2 are collected over water at 20 degree C and 785 torr, what is the mass percent yield?
(Vapor pressure of water at 20 degree C is 17.5 torr.)

2) What is the density of hydrogen, in g/L, under lab conditions? Under STP conditions?

3) Carbon monoxide can be reacted with hydrogen to produce methanol. The chemical equation for the reactions is:
CO(g) + 2H2 --> CH3OH(g)
If 52.10 L of hydrogen gas at STP is completely reacted with excess carbon monoxide gas,what volume in liters of methanol st STP will be produced?

4) The 4.25 grams of sugar (C6H12O6) reacts with sufficient oxygen to produce 3340 mL of carbon dioxide collected over water at 751.9 mm Hg at 24 degree C. Vapor pressure of water at 24 degree C is 22.4 mm Hg. What is the percent yield?
• chem 3A - GK, Friday, July 25, 2008 at 10:44pm

I am giving outlines below for solving problems #1 and #2. The others involve the same or similar principles. The general topic is reaction stoichiometry. It is hard to cover such a broad topic adequately through postings here.
Question #1
(a) Use the Ideal Gas Law to find the moles of H2
PV = nRT
P = (785 torr - 17.5 torr)/760 torr/atm = _____ atm
V = given
R = 0.0821 L.atm/K.mol
T = 20.0 + 273 = _____K
*n = (Solve for this variable)
(b) The % yield must be based on the limiting reagent.
Convert 50.0g of Al to moles (divide by 27.0 g/mol)
The mole ratio of H2 to Al is 3/2 = 1.5 according to the chemical equation.
(moles Al)(1.5) = Theoretica l Yield of H2
% yield = [calculated moles H2 / Theoretical yield](100)
Question 2
(a) Density at existing temperature:
Assume you have 1.00 mole of H2 = n
Using the Ideal Gas Law, substitute P = (pressure in atm. you calculated earlier), V = (unknown to solve for), n = 1, R = 0.0821 L.atm/K.mol, T = 293 K. Solve for V (volume in liters). Divide the molar mass of H2 by the calculated volume for 1 mole.
(b) Density at STP:
Divide the molar mass of H2 by 22.4 L.

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