Guide to Calculating Chemical Quantities
The Mole:
 A mole of any compound contains 6.02X 1023 particles; Avogardo’s Number
 1 mol = 6.02 X 1023
 The subscripts in a formula indicate the number of moles of each element in one
mole of the compound
Guide to Calculating the Atoms or Molecules of a Substance: (1 step
problem)
STEP 1: Determine the given number of moles
STEP2: Write a plan to convert moles to atoms or molecules
STEP 3: Use Avogardo’s number to write the conversion factors
STEP 4: Set up the problem to convert given moles to atoms or molecules
Example:
 Use Avogardo’s number to calculate the following: The number of Ca atoms in
3.00 mol of Ca
Given = 3.00 mol Ca
Unknown: atoms of Ca
Conversion Factor: 6.02 X 1023
1 mol
3.00 mol Ca 6.02 X 1023 atoms = 1.81 X 1024 atoms
1 mole
Molar Mass:
 The molar mass of an element is numerically equal to its atomic mass in grams
 The molar mass (g/mol) of a compound is determined by multiplying the molar
mass of each element by its subscript in the formula and adding the results
together
Guide to Calculating Molar Mass
STEP 1: Obtain the molar mass of each element (the atomic mass from the
periodic table)
STEP 2: Multiply each molar mass by the number of moles (the subscript) in the
formula
STEP 3: Calculate the molar mass by adding the masses of the elements together
Example:
 What is the molar mass of silver nitrate, AgNO3?
Ag= 107.9
N= 14.007
O= 16
1 mol Ag X 107.9 g/mol Ag = 107.9 g Ag
1 mol N X 14.007g/mol N = 14.007g N
3 mol O X 16g/mol = 48 g O
Molar Mass of AgNO3 = (107.9 g + 14.007 g + 48 g) = 169.907 g
Calculations Using Molar Mass
 The molar mass is useful as a conversion factor to change a given quantity in moles
to grams or vice versa
 The 2 conversion factors for the molar mass of NaOH (molar mass = 40.01 g/mol)
have the following forms:
40.01 g NaOH
OR
1 mol NaOH
1 mol NaOH
40.01 g/mol NaOH
Guide to Calculating the Moles of a substance from grams given or vice
versa: (1 step problem)
STEP 1: Identify the given; either moles or grams
STEP 2: Step up your T-chart
STEP 3: Determine the molar mass and write the correct conversion factor
STEP 4: Set up the problem to convert given moles to grams; or vice versa
Example:
 What is the mass in grams of 0.254 mol of Na2CO3?
Given: 0.254 mol Na2CO3
0.254 mol Na2CO3
=
Molar mass of Na2CO3 = [2(23.0 g Na) + 1(12.011 g C) + 3(16.0 g O)] = 106.011 g
Na2CO3
0.254 mol Na2CO3 106.011 g Na2CO3 = 26.9 g Na2CO3
1 mol Na2CO3
Guide to Converting Grams to Particles: (2 step problem)
STEP 1: Identify the given and the unknown
STEP 2: Find the molar mass of the given compound
STEP 3: Set up the problem to convert grams to moles
STEP 4: Add the second conversion factor to the T-chart to convert from moles to
particles (atoms or molecules)
Example:
 How many molecules of sodium carbonate are in 26.92 grams of Na2CO3?
Given: 26.92 g Na2CO3
Unknown: molecules of Na2CO3
Molar Mass of Na2CO3 = 106.011 g Na2CO3
26.92 g Na2CO3
1 mol Na2CO3
106.011 g Na2CO3
26.92 g Na2CO3 1 mol Na2CO3 6.02 X 1023 molec. Na2CO3 = 1.529 X 1023 molec. Na2CO3
106.011 g Na2CO3 1 mol Na2CO3
Percent Composition:
 Percent Composition is the percent by mass of each element in a compound
Guide to Calculating Percent Composition
STEP 1: Determine the total mass of EACH element in molar mass of a formula
STEP 2: Determine the total molar mass of the ENTIRE compound
STEP 3: Divide the total mass of EACH element by the molar mass of the entire
compound and multiply by 100
Example:
 What is the percent of Na in NaOH?
Molar Mass: Na= 1 x 23 g= 23 g Na O= 1 x 16 g= 16 g O
H= 1 x 1.008 g = 1.008 g H
Molar Mass of NaOH= 40.008 g NaOH
23 g Na
x 100 % = 57.49 % Na in NaOH
40.008 g NaOH