1. Moles and Molarity

2. I CAN calculate MOLES and MOLARITY from Formula Weight (FW) data.

3. Moles and Molarity In the lab or industrial setting, chemists must work with mass quantities that can be EASILY MEASURED. In chemistry, the unit used to measure mass of chemicals is the MOLE. Recall a mole is the MASS IN GRAMS of a substance that contains 6.022 x 10 23 particles of the substance. This number is known as AVOGADRO’S NUMBER.

4. Recall that the quantity of a substance containing Avogadro’s Number is easy to determine using the Periodic Table: For elements, the mass in grams of ONE MOLE of atoms is equal to the element’s ATOMIC MASS NUMBER. For instance, one mole of Carbon atoms = 12.011 grams of carbon atoms. Recall that certain elements, when not part of a compound, are DIATOMIC (2 atoms) and should be figured as such. For example – Oxygen, Hydrogen, Nitrogen and all of Family 17 are naturally DIATOMIC and should be written as O 2, H 2, N 2, F 2, Cl 2, Br 2, I 2 and At 2. When calculating the mass of a mole of these elements, remember it is 2 x FW for the element. For example: – a mole of Oxygen atoms is 2 x 15.999 g = 31.998 g/mole – A mole of Chlorine atoms is 2 x 35.453 g = 70.906 g/mole

5. Calculating Moles of a Substance To convert a mass, in grams, of a substance to MOLES of that substance, simply divide the mass given by the substance’s FORMULA WEIGHT (FW). moles = MASS, g FW, g/mole The abbreviation for MOLES is simply “mol”.

6. Practice Problems How many moles are contained in each of these quantities? A. 25.7 grams of Calcium Chloride, CaCl 2 ? B. 125.2 grams Mg(NO 3 ) 2 ?

7. Steps 1. Determine the formula if necessary. 2. Find the FORMULA WEIGHT (FW) of the substance. 3. DIVIDE the mass given by the formula weight and round to SF. mol = mass, g FW, g / mol

8. A. 25.7 grams of Calcium Chloride, CaCl 2 ? 1. Find the FW of CaCl 2 : 1 Ca x 40.078 g/mol = 40.078 g/mol 2 Cl x 35.453 g/mol = 70.906 g/mol + _____________ 110.984 g/mol

9. 2. Calculate the number of moles CaCl2 in 25.7 grams: mol CaCl 2 = mass, g = 25.7 g = 0.23156 mol FW, g / mol 110.984 g/mol 3. Rounded to SF = 0.232 mol CaCl 2

10. B. 125.2 grams Mg(NO 3 ) 2 ? 1. Find the FW of Mg(NO 3 ) 2 : 1 Mgx 24.305 g/mol=24.305 g/mol 2 Nx 14.007 g/mol=28.014 g/mol 6 O x 15.999 g/mol=95.994 g/mol + ____________ 148.313 g/mol 2.Calculate the moles of Mg(NO 3 ) 2 : mols Mg(NO3)2 = 125.2 g = 0.84416 g/mol 148.313 g/mol 3. Rounded to SF = 0.8442 mol Mg(NO 3 ) 2

11. ON YOUR OWN 1. How many mol of Magnesium Phosphate, Mg 3 (PO 4 ) 2 are contained in 145.5 grams? 2. How many mol of Ammonium Sulfite, (NH 4 ) 2 SO 3 are found in 66.8 grams?