Need Help with Mole Problems?
Try this ‘step-by-step’ approach, using dimensional analysis.
1.
Read the problem and figure out what information you are being given and what you
want to solve for.
 Circle the ‘given amount’ and place a ‘g’ by it.
 Circle what you want to solve for and place a ‘w’ by it.
Sample:
g
w
How many moles are there in 25 grams of copper (II) sulfate?
2.
Set up the problem, using dimensional analysis.
 Begin by writing down the given amount (with units).
 Draw in your ‘fence’.
 Set up the units for the conversion factor.
Remember
– the unit you want to solve for goes on top
– the unit you want to get rid of goes on the bottom.
 Put in the number values for the conversion factor.
Remember – If you are working with moles and grams, use the periodic table
– If you are working with moles and particles*, use Avagadro’s
Number (6.02 x 1023) [*atoms, ions, formula units, molecules]
– If you are working with moles and volume, use the molar volume
(22.4 L)
Sample:
25 g CuSO4
3.
Note: 1 Cu = 63.55 g
1 S = 32.07 g
4 O = 64.00 g
159.62 g / mol
1
mol
159.62 g
Solve the problem.
 Cancel your units to verify that you have set up the problem correctly.
 Do the math, using your calculator as needed.
 Write your answer, complete with units.
 Adjust for significant figures, as needed.
 Circle your answer.
Sample:
25 g CuSO4
1 mol
159.62 g
= 0.1566 mol
=
0.16 mol CuSO4
Need Help with Mole Problems involving mass and moles?
Try this approach, using a ‘formula’:
By definition, the molar mass (M) is the number
of grams of a pure substance found in 1 mole of
that substance.
This can be represented mathematically as…
M = mass / mole
If you know two of the three variables, you can
solve for the third – using algebra and/or the
triangle below:
For IB –
By definition, the relative molar mass (Mr) is the mass of one mole of a substance.
This can be represented mathematically as
Mr = m/n
where m = mass and n = moles
And is often rearranged to read…
n = m/ Mr where n = amount in moles and m = mass in grams
If you know two of the three variables, you can solve for the third – using algebra
and/or the triangle below:
g
m
M*
mol
Mr
To use the triangle, place your finger over the
quantity you want to solve for… then plug in
your values for the other two quantities and solve
accordingly.
To use the triangle, place your finger over the quantity you want to solve for…
then plug in your values for the other two quantities and solve accordingly.
Need Help with Mole Problems
involving number of particles and moles?
Try this approach, using a ‘formula’:
Practice Problems:
1.
2.
3.
By definition, Avagadro’s number is the number
of particles of a pure substance found in 1 mole
of that substance.
This can be represented mathematically as…
NA = 6.02 x 1023 = N / n
where N = number of particles
and n = amount in moles
If you know two of the three variables, you can
solve for the third – using algebra and/or the
triangle below:
N
6.02 x
1023
n
To use the triangle, place your finger over the
quantity you want to solve for… then plug in
your values for the other two quantities and solve
accordingly.
n
4.
5.
6.
How many moles is 2.80 x 1024 atoms of silicon?
How many molecules are in 1.14 mol SO3?
How many atoms are in 1.14 mol SO3?
Hint – Start with your answer from 2 … then figure
out how many atoms are in 1 molecule (1 molecule
SO3 = ? atoms) and use this as a conversion
factor.
Find the molar mass of PCl3.
Calculate the mass, in grams of 2.50 mol of iron(II)
hydroxide - Fe(OH)2
Calculate the number of moles in 75.0 g of
dinitrogen trioxide - N2O3
Answers:
1. 4.65 mol Si
2. 6.86 x 10 23 molecules SO3
3. 2.75 x 1024 atoms (S & O)
4. 137.5 g/ mol PCl3
5. 225 g Fe(OH)2
6. 0.987 mol N2O3