Mass Measurements In Chemistry Calculating Formula Weight

I CAN calculate the formula weight (molar mass) of a compound from it’s formula.

We’ve talked about the mass of atoms in terms of one or two….but those masses are very small. To be practical for laboratory work, a standard of measure had to be derived so that mass quantities could be easily measured. To that end, scientists created a unit called the ATOMIC MASS UNIT used to measure the mass of subatomic particles.

For AMU, protons and neutrons = 1 amu (u). Scientists looked for a connection between the mass of a single atom in AMUs and the same mass in GRAMS. For instance, Carbon-12 has 6 P+ and 6 N o, so it’s mass in AMUs is 12 u. To equal 12 grams, it was found that x atoms were required. As it turns out, ANY element’s mass in AMUs will require x atoms to has the same mass in GRAMS! x is called AVAGADRO’S NUMBER.

The quantity of matter containing Avagadro’s Number of particles, such as atoms (element), molecules (covalent) or formula units (ionic), is known as a: Mole When working in a lab setting, the MOLE is used to determine quantities of chemicals.

Mass Calculations and Chemical Formulas Chemical Formulas represent the ELEMENTS and the NUMBER OF ATOMS of each that compose a compound. – Subscripts in formulas relate to the element immediately IN FRONT of them. – If a formula contains a PARENTHESIS followed by a subscript, the subscript multiplies ALL elements inside.

EXAMPLE Ca 3 (PO 4 ) 2 3 atoms Ca 2 atoms P 8 atoms O

Calculating Formula Weight (Molar Mass) Formula Weight (Molar Mass) of an element or compound is the MASS in GRAMS of ONE MOLE of the element or compound. For an element, the FW is the ATOMIC MASS from the periodic table, or for a particular ISOTOPE. For a compound, the FW is found by counting the number of each kind of atom. Multiply the number of atoms X the Atomic Mass Number of the element (from the PT). Add the results and round to significant figures.

Example What is the formula weight of Calcium Carbonate? The formula is: CaCO 3 Count the number of moles of EACH ATOM and multiply by the ATOMIC MASS NUMBER from the PT: From the Formula From Periodic Table 1 mole Ca X g = g  this one only has 2 SF past decimal 1 mole C X g = g  this one has 3 SF past decimal 3 moles O X g = g  this one has 4 SF past decimal ________________ = g/mole  final answer must be rounded to 2 SF = g/mole past the decimal. ******STOP HERE FOR TODAY*********

Practice Problem Calculate the Molar Mass (FW) of Lithium Hydrogen Sulfate, LiHSO 4. 1 mole Li x g= g 1 mole H x g = g 1 mole S x 32.06g =32.06 g 4 moles O x g= g g/mole = g/mole

Hydrates Certain ionic compounds trap water molecules in their structure when they form. These are known as HYDRATES. Hydrates are written in this manner: Compound Formula Dot Number of Water Molecules CuSO 4 5H 2 O Hydrates are named as: Compound Name, a Prefix for the number of water molecules, then the word Hydrate. When calculating the formula weight of a hydrate, just include the MASS OF THE NUMBER OF WATER MOLECULES in the total.

Practice Problem What would be the formula weight of Magnesium Chlorate Pentahydrate? Formula = Mg(ClO 3 ) 2  5H 2 O 1 mole Mg x g= g 2 moles Cl x g= g 6 moles O x g= g 10 moles H x g= g 5 moles O x g= g g/mole

PRACTICE PROBLEMS Complete the following FORMULA WEIGHT (MOLAR MASS) calculations ON YOUR OWN PAPER! 1. Find the FORMULA WEIGHT of Aluminum Sulfate, Al 2 (SO 4 ) What is the MOLAR MASS of Glucose, C 6 H 12 O 6 ? 3. Find the FORMULA WEIGHT of: Sodium Potassium Permanganate Tetrahydrate NaK(MnO 4 ) 2  4H 2 O 4. What is the MOLAR MASS of Ammonium Phosphate Dihydrate.

KEY

Percentage Composition An Analytical Chemistry Technique

I CAN calculate the PERCENTAGE COMPOSITION of a compound from Formula Weight data.

Percentage Composition An analytical chemistry technique used in studying composition of substance is Percentage Composition. In Percentage Composition, Formula Weight (Molar Mass) data is used to determine the overall contribution the mass of each element makes to the mass of a compound.

Calculating Percentage Composition Determine the FW (MM) of the compound. Divide the TOTAL MASS of each element by the FW for the compound. Multiply the results by 100 to convert to a percentage. Add all percentages to check the answer…it should be VERY CLOSE to 100% (may be slightly off due to rounding).

Practice Problem What is the percentage composition of Magnesium Hydroxide, Mg(OH) 2 ? STEP 1. Calculate the FW: 1 mole Mg X g = g/mole 2 moles O X g = g/mole 2 moles H X g = g/mole + _____________ g/mole (Since this isn’t the last step, don’t round to SF here)

STEP 2. Divide EACH ELEMENT’S mass by the FW: Mg = g X 100 = % g O = g X 100 = % g H = g X 100 = % g STEP 3. Add all the %s : = % = 100%

ASSIGNMENT For each of the 4 problems on yesterday’s assignment, calculate the PERCENTAGE COMPOSITION of each element!