Chemistry Reference
In-Depth Information
2H
2
O
hydrogen + oxygen
water
x dm
3
30.0 dm
3
Now we need to convert the volume of the oxygen to number of moles,
by dividing by the molar volume of a gas at STP, 22.4 dm
3
/mole.
2H
2
+ O
2
2H
2
+ O
2
2H
2
O
hydrogen + oxygen
water
x dm
3
30.0 dm
3
# of moles of oxygen =
volume of gas at STP
molar volume of gas
30.0 dm
3
22.4 dm
3
/mole
= 1.34 moles
=
Next, we need to use the molar ratio, given by the coefficients, to de-
termine the number of moles of hydrogen that will react with 1.34 moles of
oxygen. The molar ratio is 2:1, which means that it would take 2 moles of
hydrogen to react completely with 1 mole of oxygen. It follows that it will
take 2.68 moles of hydrogen to react with 1.34 moles of oxygen, as shown
by this ratio:
hydrogen
oxygen
coefficients
2
x
1
1.34 moles
=
# of moles
Cross-multiplying, we get x = 2.68 moles.
All that is left for us to do is Step 5, to convert the number of moles to
the desired quantity, which, in this case, is dm
3
. To do this, we have to mul-
tiply the number of moles of hydrogen (2.68 moles) by the molar volume of
a gas at STP.
2H
2
O
hydrogen + oxygen
water
x dm
3
2H
2
+ O
2
30.0 dm
3
# of moles of oxygen =
volume of gas at STP
molar volume of gas
30.0 dm
3
22.4 dm
3
/mole
= 1.34 moles
=
Volume of H
2
= # of moles of H
2
at STP × molar volume of gas
= 2.68 moles × 22.4 g/mole = 60.0 dm
3
So it takes 60.0 dm
3
of hydrogen to react completely with 30.0 dm
3
of
oxygen, when both gases are at STP. Now, did you figure out what the short-
cut is? In Step 3, we converted the volume of oxygen to moles by dividing
by 22.4 dm
3
/mole. In Step 5, we converted the number of moles of hydro-
gen to volume by multiplying by 22.4 dm
3
/mole. These steps are opposite
operations, so they actually cancel each other out! This means that we can
