Chemistry

posted by Kay

Stoichiometry-I have the basic strategy, but am still stuck on how to do them.example...propane burns in oxygen according to the following reaction
C3H8 + O2 produces CO2 + H20
a. How many moles of CO2 are formed when 2.8 g of propane reacts?

b. If 0.45 g of oxygen is used in the burning of propane, how many moles of each CO2 and H2O are produced?

1. DrBob222

Here are the steps (at the last of the post)
Stoichiometry-I have the basic strategy, but am still stuck on how to do them.example...propane burns in oxygen according to the following reaction
C3H8 + O2 produces CO2 + H20
a. How many moles of CO2 are formed when 2.8 g of propane reacts?

b. If 0.45 g of oxygen is used in the burning of propane, how many moles of each CO2 and H2O are produced?

Step 1. Write and balance the equation.
C3H8 + 5O2 ==> 3CO2 + 4H2O

Step 2. Convert 2.8 g propane to moles. moles = g/molar mass.
2.8/44 = 0.636 moles propane

Step 3. Using the coefficients in the balanced equation, convert moles of what you have (in this case propane) to moles of what you want (in this case CO2).
moles CO2 = mols propane x (3 moles CO2/1 mol C3H8) = 0.636 x (3/1) = 0.19 moles CO2.[Note how the unit you don't want cancels and leaves the unit you want to convert to).

Step 4. Most problems ask for grams of the product (this one--part a-- just asked for moles) but if you want grams, then grams CO2 = moles x molar mass = 0.19 x 44 = 8.4 grams. You need to go through this because I estimated the molar masses and rounded here and there. At the end you need to round the answer to 2 significant figures (in line with the two s.f. in 2.8 g propane).
Part b is done the same way but that question asks for grams so you will need to use the fourth step. This procedure also will do limiting reagent problems if you throw in an extra step or two as sub parts of steps 2 and 3.

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