gravimetric stoichiometry

bluedicius · #1 Nov 22, 2004, 02:13 PM

Powdered zinc metal reacts violently with S8 when heated. Predict the mass of sulfur required to react with 25g of zinc.

Bauxite contains aluminum oxide, which is decomposed using electricity to produce aluminum metal. What mass of aluminum metal can be produced from 100g of aluminum oxide?

Determine the mass of oxygen required to completely burn 10.0g of propane.

urmod4u · Nov 23, 2004, 05:42 AM

Zn + S -> ZnS
Note that I didn't write S8, S8 is only one of the forms of sulphur and it doesn't matter for the calculations anyway.
Atomics weights of Zn and S are 65.41 and 32.07 resp.
So, 25g of Zinc will react with 25*32.07/65.41 = 12.2573 g of Sulphur.
*****
One mole Al2O3 weights 2*26.98 + 3*16 = 101.96 g and contains 2*26.98 = 53.96g Al.
Dissociation of 100g Al2O3 will give 100*53.96/101.96 = 52.9227 g Al. The remainder (100-52.9227=47.0773g) is the Oxygen of course which is also found by 100*3*16/101.96= 47.0773.
******
Propane C3H8 burns following the equation:
C3H8 + 5 O2 -> 3 C02 + 4 H2O
Propane's weight is 3*12 + 8*1 = 44, O2's weight is 2*16 = 32
One mole propane needs five moles oxygen to burn, that is 5*32/44 = 3.6364 times its weight. So, 10 g propane needs 36.364 g of oxygen to burn.

bluedicius · Nov 24, 2004, 05:34 PM

Thank you so much for all your help. Are there any easy to follow steps or a procedure to follow when doing stoichiometry questions and formulas like these? How do you know you have the right chemical formula? That its balanced?
Thanks

urmod4u · Nov 25, 2004, 03:34 PM

The answer to the last question is: you must have the same number of each element's atoms on both sides of the equation. You must also respect the valency of each component. It is very simple maths. Then it's balanced.
The other questions are a bit more difficult to answer.
You must know the valence(s) of each element, and how each element behaves in chemical reactions.
You must know of each combination what the properties are, and how they behave.

dashredda · Sep 29, 2007, 02:31 PM

How many moles of lithium hydroxide is required to completely neutralize 2.39 mol of oxalic acid?

1Sr(OH)2 + 2HClO4 = 1Sr(ClO4)2 + H20



At Ask Me Help Desk you can ask questions in any topic and have them answered for free by our experts. To ask questions or participate in answering them you must register for a free account. By registering you will be able to: Get free answers from experts in any of our 300+ topics. Accept money for answers that you provide. Communicate privately with other members (PM). See fewer ads. Sign Up!