ps12

Key Concept: The Mole (436 only)

Sample Questions:

(JUNE 2000 430): Iron ore, Fe₂O₃, is refined to iron, Fe, by reacting the iron ore with coke, C, according to the balanced equation:

Fe₂O₃+ 3 C --> 2 Fe + 3 CO

What mass of coke is required for every 6 moles of Fe₂O₃ refined?

2 g
18 g
216 g
670 g

(JUNE 2000 430): In a car battery lead, Pb. Lead oxide, PbO₂, and sulfuric acid, H₂SO₄, react to produce lead sulfate, PbSO₄ and water according to the following balanced equation:

Pb + PbO₂ + 2 H₂SO₄-->2 PbSO₄+ 2 H₂O

What mass of Pb would react to produce 28.71 g of PbSO₄?

(JAN 2000 430): If 60 g of propane (C₃H₈) is burned on a stove, carbon dioxide and water are produced, as shown below:

C₃H₈+ 5 O₂ --> 3 CO₂+ 4 H₂O

What mass of CO₂ will be released into the environment?

(JUNE 1999 436): A technician measures out 227.8 g of magnesium nitrate, Mg(NO₃)₂. Now many moles (mol) are contained in this sample of magnesium?

0.59 mol
0.65 mol
1.54 mol
1.70 mol

(JUNE 1995 436): Burning ethane, C₂H₆, in air containing oxygen, O₂, produces carbon dioxide, CO₂, and water, H₂O. You are to produce 32 moles of carbon dioxide.

What mass of oxygen is required?

Answer: Nice question because you first have to create a balanced equation for what is described!

1 C₂H₆+ 3.5 O₂--> 2 CO₂+ 3 H₂O

(3.5/x) = (2/32) x = 56 moles of O₂.

56 * 32 g/mole = 1792 g

Notes: Never base a ratio on grams!!!!!

Although the above flow chart seems like a page out of the instruction manual for assembling a space shuttle, it is actually simpler than it seems. Keep the following in mind:

the coefficients (big numbers) in front of the compounds represent moles.

A mole is a group of molecules (6.02 X 10 ²³). It is these moles that are proportional to each other. For example in 1Pb + PbO₂ + 2 H₂SO₄-->2 PbSO₄+ 2 H₂O, the equation is telling you that for every 1 mole of Pb that reacts, 2 moles of H₂O form. Of course if you had 30 moles of Pb reacting 2 (30)= 60 moles of water would form,.

But if you are not given moles in the problem (see #2 above in which they have 28.71 g of PbSO₄forming), you have to convert that to moles. The chart above reminds you that in order to convert grams to moles you have to divide by the number of grams in a mole (molar mass from periodic table), So for example 28.71 g/ (207+32+4*16) = 0.095 moles of PbSO₄.

The answer from a ratio always yields moles. So back to example 2:

1Pb + PbO₂ + 2 H₂SO₄-->2 PbSO₄+ 2 H₂O. The ratio of Pb (that's the one they ask for) to PbSO₄ is 1:2.

So (1/x)=(2/0.095); x = 0.0473 moles of Pb.

But if a problem asks for grams, you must convert to grams by multiplying moles by the number of grams in a mole (molar mass). Example 0.0473 moles of Pb*(207 g/mole) = 9.8 g of Pb