Solving Stoichiometry Problems

Question

Predict the mass of hydrogen gas produced when 2.73 g of aluminum reacts in a single replacement reaction with excess sulphuric acid.

Answer 1

Looking at the conjugate base of this acid bisulfate, HSO4-2, and SO4- and acknowledging that this is a strong acid we can predict and follow what this reaction would look like with replacement. Strong acids completely ionize H+ in aqueous solution. They push off all of their Hydrogens whereas weak acids don't necessarily give up all of their Hydrogens component.

So by definition we can predict that Sulfuric acid will give off all of its Hydrogens creating space for Aluminum. The conjugate being bisulfate HSO4- sulfate SO4-2 Whether in one or two steps all of the Hydrogen will be removed from this strong acid.

This is the prototype of a replacement rxn.

AX +B==> A + BX

The part that is being removed is hydrogen so that the sulfate component can combine with Aluminum. And it states a SINGLE replacement reaction or all at once.

===>AlSO4

Now you must balance to put the anion and cation together so that there is no net charge.

Sulfates charge is: -2

Aluminum charge is: +3 it sits in the 3rd group so it takes on a +3 charge

So you will have: Al2(SO4)3

The components of each go together to balance the -6 from the Sulfate and the +6 from the aluminum, leaving the net charge to be 0.

In the problem you are told that it is gas. Gases hang out in their diatomic form so two atoms double bonded would look like H2.

Al + H2SO4==> Al2(SO4)3 + H2

Now balance the equation by putting a 2 in front of Aluminum, a 3 in front of sulfuric acid and a 3 in front of diatomic hydrogen, H2.

Now you have:

2Al + 3H2SO4==> Al2(SO4)3 + 3H2

Now with this balanced equation, find out what this means for grams and moles of substance.

Based on the equation, 2 moles of Aluminum react with 3 moles of Hydrogen.

Look at what you have been given:

2.73g of Aluminum is how many moles?

Use Aluminum's atomic weight to find out:

2.73g Al *1mol/26.98g

Notice how it is written so grams cancel top to bottom and you will be left with moles.

0.1012 moles of Aluminum.

Now based on the balanced equation, what would this mean for substance of H2 or Hydrogen?

2moles of Al==> 3 moles H2

0.1012 moles of Al==> X moles of H2

Now cross multiply divide and solve for X:

2X= 3*0.1012

X= (3*0.1012)/2 = 0.1518 moles of Hydrogen.

Now with moles take this to grams:

0.1518 mol of H2* 2.02g/1 mol, moles cancel top to bottom and you will be left with grams at:

0.3066 grams of H2 or Hydrogen or

0.307 grams of H2 or Hydrogen gas.