STAAR Ladder to Success Rung 8. How do chemists define a mole? Example #1: A sample consists of 6.85 x 10 20 atoms of carbon. How many moles does the.

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Presentation transcript:

STAAR Ladder to Success Rung 8

How do chemists define a mole? Example #1: A sample consists of 6.85 x atoms of carbon. How many moles does the sample contain?

Example #2: Another sample consists of 2.58 mol of water. What is the number of water molecules in the sample?

Example #3: How many atoms are in 2 moles of He?

Example #4: Find the number of chlorine ions in 5 grams of CaCl 2.

Example #5: How many molecules are in 7.1 grams of water?

Calculating percent composition, empirical and molecular formulas.

A. Percentage Composition the percentage by mass of each element in a compound

Example #1: Calculate the percent composition of each element in MgCl 2

B. Empirical Formula C2H6C2H6 CH 3 reduce subscripts Smallest whole number ratio of atoms in a compound

B. Empirical Formula 1. Find mass (or %) of each element. 2. Find moles of each element. 3. Divide moles by the smallest # to find subscripts. 4. When necessary, multiply subscripts by 2, 3, or 4 to get whole #’s.

Example #1: Find the empirical formula for a compound that is 79.8% C and 20.2% H.

C. Molecular Formula “True Formula” - the actual number of atoms in a compound CH 3 C2H6C2H6 empirical formula molecular formula ?

C. Molecular Formula 1. Find the empirical formula. 2. Find the empirical formula mass. 3. Divide the molecular mass by the empirical mass. 4. Multiply each subscript by the answer from step 3.

Example #1: The empirical formula for a molecule is NH 2. If its molecular weight is 32 amu, what is the compound’s molecular formula?

mass is neither created nor destroyed in a chemical reaction Law of Conservation of Mass 4 H 2 O 4 H 2 O 4 g32 g 36 g total mass stays the same atoms can only rearrange

Balancing Equations Example #1: H 2 + N 2  NH 3

Stoichiometry Example #1: Suppose 8.75 g of propane react with oxygen gas to produce carbon dioxide and water. How many grams of water are produced? C 3 H 4 + 5O 2  3CO 2 + 4H 2 O

Stoichiometry Example #2: What is the limiting reagent when 36 g of CH 4 when it reacts with 98 g O 2 to produce carbon dioxide and water? CH O 2  CO 2 + 2H 2 O

% Yield Example 1: You calculate a theoretical yield of 55 g of water but the actual yield from your experimentation was 48.8 g of water. What is the percent yield?