S. 5.2 – The Mole
Molar mass (g/mol):
|Element |Atomic mass shown on Periodic |Molar mass of element | | |table | | |N |14.0067 |14.0067 g | |F |18.998403 |18.998403 g | |P |30.97376 |30.97376 g | |V |50.9415 |50.9415 g | |Ni |58.69 |58.69 g |
Finding the molar mass of a compound.
- Look up the molar masses of the elements within the compound using the periodic table. - Add up the masses and express in grams.
- Round the molar masses to two decimal places. The more decimal the places the more precise your answer is.
1) Determine how many atoms of each element are within the compound. 2) Determine the molar mass of each element.
3) Multiply the molar mass of each element by how many atoms of that element are within the compound. 4) Add up the numbers produced in step three.
Ex. 1: Calculate the molar mass of C6H4Cl2. 1) 6 C atoms, 4 H atoms, 2 Cl atoms
2) C = 12.01 g, H = 1.01 g, Cl = 35.45 g.
3) C = 6 × 12.01 g = 72.06 g,
H = 4 × 1.01 g = 4.04 g,
Cl = 2×35.45 g = 70.9 g.
4) 72.06 g + 4.04 g + 70.9 g = 147 g = molar mass = 1 mol C6H4Cl2
It is easier to set up the equation as: 6 C = 6 × 12.01 g = 72.06 g
4 H = 4 × 1.01 g = 4.04 g
2 Cl = 2 × 35.45 g = 70.9 g
molar mass = 147 g C6H4Cl2
Ex. 2: Calculate the molar mass of Mg3(PO4)2
We have 2 PO4
Ex. 3: Calculate the molar mass for Cu(NO3)2·6H2O
- Pg. 80, # 6 – 7.
Calculations Relating the Number of Moles and The Mass of Substance.
Why we do calculations.
- THE TEACHER MAKES US.
- Can determine the mass produced in a reaction.
- Can determine the amounts of reactants required to react so neither is in excess. Chemicals are expensive and a waste is no good.
Two Possible Conversion Factors.
1) Find the molar mass of the compound.
2) Determine the initial, conversion statement equation. 3) Determine the unknown (grams or moles)
Ex. 1: What is the mass of 5.21 mol NH3
1) Molar mass of NH3 =
2) Mass of NH3:
Ex. 2: What is the mass of 1.46·102 NaBr?
1) Molar mass of NaBr =
2) Mass of NaBr:
Ex. 3: Determine the number of moles of H2O there are in 1000 g of H2O. 1) Molar mass of H2O is
2) Moles of H2O is:
Ex. 4: Determine the number of moles of C2H5OH there are in 355 g C2H5OH. 1) Molar mass of C2H5OH is
2) Moles of C2H5OH is:
Finding molar mass from amount of substance and mole of substance. - Calculated by dividing the mass of a substance by the number of moles contained in the substance.
Ex. If 0.140 mol of acetylene gas has a mass of 3.64 g, what is...
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