 # Mathematical Chemistry

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Mathematical Chemistry
Chemical Mathematics

Chapter 7 Homework (II) Due Monday, November 3rd Pgs. 236-238
Problems 30, 33, 40, 45, 46, 47, 50

Molar Mass

Calculate the molar mass
We will learn how to… Calculate the molar mass Calculate the percent composition by mass of a compound.

Formula Mass The formula mass of any compound or polyatomic ion is the sum of the average atomic masses of all the atoms represented in the formula.

Formula mass of H2O is u. The mass of a single water molecule is u. The mass of a mole of water (6.022 x 1023 molecules of water) is g.

Sample Problem Find the molar mass of potassium chlorate, KClO3. H2SO4
Ca(NO3)2 PO43- g/mol g/mol g/mol 94.97 g/mol

One mole equals two things…
One molar mass (MM) of a compound. Avogadro’s number of molecules.

What is the molar mass of barium nitrate, Ba(NO3)2?
Sample Problem What is the molar mass of barium nitrate, Ba(NO3)2? g.

Molar Mass is a Conversion Factor.
Molar mass of a compound is a conversion factor that relates mass in grams to number of moles and vice-versa.

Conversion Factor How many eggs are there in 3 dozen? If one mole of water has a mass of 18. g, how much mass does three moles of water have?

A really important formula!

Practice Problems What is the molar mass of Copper (II) nitrate.
g What is the mass in grams of 6.25 mole of Copper (II) nitrate. 1170 g.

(cont.) How many molecules are contained in: 25.0 g H2SO4?
125 g. of sugar (C12H22O11) 1.53 x 1023 molecules of Sulfuric Acid 2.20 x 1023 molecules of Sugar

Percent Composition

Percent Composition The percent composition of a compound is the percent by mass of each element in the compound.

Percent Composition

Find the percent composition to 4 significant figures...
Practice Find the percent composition to 4 significant figures... PbCl2 74.50% Pb and 25.50% Cl

What’s the percent water in…
ZnSO4•7H2O 43.85%

Find the percent composition of (NH4)2CO3?

Practice Problem 7-9 Ibuprofen, C13H18O2, is the active ingredient in many nonprescription pain relievers. Its molar mass is g/mol. If the tablets in a bottle contain a total of 33 g. of ibuprofen, how many moles of ibuprofen are in the bottle? 0.16 mol

9.6 x 1022 Molecules Practice Problem 7-9
Ibuprofen, C13H18O2, is the active ingredient in many nonprescription pain relievers. Its molar mass is g/mol. How many molecules of ibuprofen are in the bottle? 9.6 x 1022 Molecules

Practice Problem 7-9 Ibuprofen, C13H18O2, is the active ingredient in many nonprescription pain relievers. Its molar mass is g/mol. What is the total mass in grams of carbon in 33 g. of ibuprofen? 25 g

7.4 Determining Chemical Formulas
The simplest formula or empirical formula consists of the symbols for the elements combined with subscripts showing the smallest whole-number ratio of the atoms.

Example Diborane has a molecular formula and in reality exists as B2H6. The empirical formula is BH3. Both are 78% Boron and 22% Hydrogen.

Simplest Formula (from weight percents.)
Step 1: Take percentages and convert to a total of 100 g. Diborane is 78% by weight Boron and 22% by weight hydrogen. 100 g would be 78 g. of Boron and 22 g. of Hydrogen.

Simplest Formula (cont.)
Step 2: Determine moles.

Simplest Formula (cont.)
Step 3: Determine simplest whole number molar ratio. Divide through by the smallest number.

Simplest Formula (cont.)
7.1 mol of Boron; 22 mol H 1.0 mol B : 3.1 mol H Empirical formula is BH3

Practice Problem Analysis shows a compound to contain 26.56% potassium, 35.41% chromium, and % Oxygen by weight. Find the simplest formula for this compound. K2Cr2O7.

Molecular Formula The simplest or empirical formula may or may not the correct molecular formula. Is the correct (real) formula BH3, B2H6, B3H9 ? Need to know the molecular mass of the formula.

Molecular Formula (cont.)
If the formula mass of the empirical formula equals the known molecular mass, then it is the molecular formula.

Molecular Formula (cont.)
If not, there is a whole number multiplier, x, for all of the subscripts.

Empirical formula is BH3
The BH3 has a formula mass of u. The known formula mass for Diborane is u.

(BH3)(2) = B2H6

Determine the molecular formula of a compound having the simplest formula of CH and a formula mass of u.

Review 4.04 g of N combine with g O to produce a compound with a formula mass of u. What is the molecular formula? N2O5