Chemistry Reference
In-Depth Information
Not all of these problems will deal with one product and one reactant.
Sometimes, everything that you need to work with is on the same side of
the reaction arrow. Let's try another problem of this type.
Example 2
How many grams of water would be required to react completely
with 3.25 grams of sodium in a reaction that produces only hydrogen
gas and sodium hydroxide?
Well, you weren't given a balanced chemical reaction this time, so that
is the place to start. You do have enough information to produce the fol-
lowing word equation (see Lesson 6-3).
sodium + water
hydrogen and sodium hydroxide
You can then use your knowledge of elemental symbols (Lesson 3-2),
writing chemical formulas (Lesson 5-2), and balancing chemical reactions
(Lesson 6-1) to turn the word equation into a balanced chemical reaction.
water + sodium
hydrogen + sodium hydroxide
becomes
2H
2
O + 2Na
H
2
+ 2NaOH
Now, let's go back to the original problem and use the given informa-
tion to label the equation.
How many grams of water would be required to react completely with
3.25 grams of sodium in a reaction that produces only hydrogen gas
and sodium hydroxide?
2H
2
O + 2Na
H
2
+ 2NaOH
x g 3.25 g
Notice that you were neither given, nor asked for, a quantity of hydro-
gen or sodium hydroxide. These were mentioned only for the sake of pro-
ducing a balanced chemical reaction. Therefore, you can treat them in the
same way that we treated oxygen, the excess reactant, in the last problem.
Now, you need to convert 3.25 grams of sodium into moles by dividing
by the molar mass of sodium (23.0 g/mole), which you get from the peri-
odic table.
2H
2
O + 2Na
H
2
+ 2NaOH
x g
3.25 g
mass of sample
molar mass
3.25 g
23.0 g/mole
= 0.141 moles
# of moles of Na =
=
