Chapter 3: Calculations with Chemical Formulas

Renee Y. Becker

Valencia Community College

CHM 1045

Atomic and Molecular Mass

• The molecular mass is the sum of the masses of the atoms making up the molecule (units amu) (molecular compounds only!)

calculate the molecular mass:

H2O

C2H4O2

• Formula Mass is the sum of atomic masses of all atoms in a formula unit of any compound, molecular or ionic (units amu)

calculate the formula mass:

NaCl

MgCO3

Formula Mass

Avogadro

• One mole of a substance is the gram mass value, molar mass, equal to the amu mass of a substance

• Molar mass, (MM), is in units grams/mole (conversion factor for gmol and molg)

• Avogadro’s Number (NA = 6.022 1023 particles) is the numerical value assigned to the unit, 1 mole

Example 1: Molar Mass

Calc. the molar mass:

Fe2O3

C6H8O7

C16H18N2O4

Example 2: Avogadro’s Number

Li2SO4

1. How many Li2SO4 units are in 3 moles of Li2SO4?

2. How many Li atoms are in 3 moles of Li2SO4?

3. How many S atoms are in 3 moles of Li2SO4?

4. How many O atoms are in 3 moles of Li2SO4?

Example 3: Molesg Calculate

2.35 moles C g C

4.67 x10-3 moles H2O g H2O

83.2 moles CaO g CaO

Example 4: g Moles

Calculate

45.8 g Fe moles Fe

6.54 x104 g C2H4 moles C2H4

Stoichiometry

• Stoichiometry: Relates the moles of products and reactants to each other and to measurable quantities

Example 5: Stoichiometry

2 NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l)

1. How many moles of Cl2 are needed to react with 2 moles of NaOH?

2. How many moles of Cl2 are needed to react with 3 moles of NaOH?

3. How many moles of NaCl are formed by this reaction?

4. If you start with an excess of Cl2 and 5 moles of NaOH, how many moles of H2O can you produce?

Example 6: Stoichiometry

2 NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l)

1. How many moles of NaOH are needed to react with 25.0 g of Cl2?

2. How many grams of NaOH are needed to react with 25.0 g of Cl2?

Example 7: Stoichiometry

• Let’s make up some more examples

2 NaOH(aq) + Cl2(g) NaOCl(aq) + NaCl(aq) + H2O(l)

Stoichiometry

• Yields of Chemical Reactions: If the actual amount of product formed in a reaction is less than the theoretical amount, we can calculate a percentage yield.

% Yield = Actual yield X 100%

Theoretical yield

Example 8: Stoichiometry

• CH2Cl2 is prepared by reaction of CH4 with Cl2 giving HCl. How many grams of CH2Cl2 result from the reaction of 1.85 kg of CH4 if the yield is 43.1%?

CH4(g) + 2 Cl2(g) CH2Cl2(l) + 2 HCl(g)

Stoichiometry

• Limiting Reagents: The extent to which a reaction takes place depends on the reactant that is present in limiting amounts--the limiting reagent.

– I need to make a fruit salad that is 1/2 apples and 1/2 oranges. I have 10 apples but only 7 oranges. What is the limiting fruit? How many apples and oranges can I use?

Example 9: Stoichiometry

• Limiting Reagent Calculation: Lithium oxide is a drying agent used on the space shuttle. If 80.0 kg of water is to be removed and 65 kg of lithium oxide is available, which reactant is limiting?

Li2O(s) + H2O(l) 2 LiOH(s)

Li2OMM = 29.88 g/mol

H2OMM = 18.02 g/mol

Example 10: Stoichiometry

K2PtCl4 + 2 NH3 Pt(NH3)2Cl2 + 2 KCl

• If 10.0 g of K2PtCl4 and 10.0 g of NH3 are allowed to react:

a) which is limiting reagent?

b) how many grams of the excess reagent are consumed?

c) how many grams of cisplatin are formed?

K2PtCl4 = 415.08 g/mol NH3 = 17.04 g/mol

Pt(NH3)2Cl2 = 300.06 g/mol

Percent Composition

• Percent Composition: Identifies the elements present in a compound as a mass percent of the total compound mass.

• Empirical Formula: Determined from data about percent composition, tells only the smallest whole number ratio of atoms in a compound.

• Molecular Formula: Tells actual numbers of atoms in a molecule, can be same as empirical formula or a multiple of it.

Example 11: Percent Composition

What is glucoses, C6H12O6, empirical formula, and what is the percentage composition of glucose?

Example 12: Percent Composition

• Saccharin has the molecular formula C7H5NO3S, what is its empirical formula and the percent composition?

Example 13: Empirical Formula

• A compound was analyzed to be 82.67% carbon and 17.33% hydrogen by mass. The molar mass is 58.11 g/mol.

– What is the empirical formula and molecular formula?