Module 7

Question 1

Elemental analysis of binary compounds
(Finding empirical formula given the element percentages of a molecule)
Ex) Empirical formula of a molecule made from 70% Fe and 30% O.

Answer 1

1. Multiply percentages by 100g
2. Divide each by their molar mass
3. Use whoever has the smallest quotient (from 2) and divide each by it.
4. Multiply by whole numbers, if any of the quotients (from 3) are not whole numbers. If they are, then that is the empirical formula.

Question 2

Finding mole ratios from chemical formulae
(given a formula and moles of an element w/in formula)

Answer 2

Established:
moles of Element1 / moles of Element 2 = atoms of E1 / atoms of E2

Ex) C9H13O3N, 4.15 moles H in sample, How many moles of O in sample?

1. Set up ratio based on above formula.
   atoms(moles) O / atoms(moles) H = 3/13
2. Isolate the moles you want.
   moles of O = moles of H * (ratio(3/13))
3. Multiply

Question 3

Calculate and use the molar mass of diatomic elements

Answer 3

1. Remember that it’s a diatomic element.
2. Multiply molar mass of element by 2
3. Multiply that molar mass by the given moles
4. Round to sig figs

Question 4

Finding the ratio of an element’s mass to the rest of the compound (given molecular formula)

Answer 4

1. Multiply the number of atoms with element’s atomic mass
2. Find the molar mass of the rest of the compound
3. Divide the product from 1 with the sum of 2.