Chemistry Reference
In-Depth Information
Once you know that you are starting with 0.141 moles of sodium, you
use the molar ratio given by the coefficients to determine the number of
moles of water you will need to react completely with the sodium. Again,
the ratio between the two substances in question is 2:2, which simplifies to
1:1. This means that it takes 1 mole of water to react completely with 1
mole of sodium. It would take 0.141 moles of water to react completely
with 0.141 moles of sodium.
Then, you only have to multiply the number of moles of water with the
molar mass of water (18.0 g/mole) to find the mass of water required.
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 =
=
Mass of water = # of moles × molar mass = 0.141 moles × 18.0 g/mole = 2.54 g
So, it would take 2.54 g of water to react completely with 3.25 g of
sodium.
I don't want to give you the impression that the molar ratio given by
the coefficients will always be 1:1, so let's try another example, where the
ratio is not 1:1.
Example 3
How many grams of water are produced as 3.00 g of carbon dioxide
are generated from the combustion of methane (CH
4
)?
This question didn't give you a balanced chemical reaction to start with.
You must use your knowledge of combustion reactions to fill in the infor-
mation. As you surely know by now, having studied Lesson 6-2, the typical
combustion reaction takes on the following form:
hydrocarbon + oxygen
water and carbon dioxide
That makes the word equation for this specific combustion reaction:
methane + oxygen
water and carbon dioxide
Use your knowledge of chemical formulas to change the word equa-
tion into the following balanced equation:
CH
4
+ 2O
2
2H
2
O + CO
2
Now let's insert this equation into the problem and label it based on
the information given.
