Unit 3 - Quantitites In Chemical Reactions

Question 0

How do you calculate percent composition?

Answer 0

Divide the molar mass of the element (multiplied by number of atoms of that element) by the total molar mass of the molecule and multiply by 100

Question 1

What is the difference between empirical and molecular formula?

Answer 1

The empirical formula is the simplest whole number ratio and the molecular formula tells you exactly how many atoms are in the molecule.

Question 2

Formulas for ionic compounds are always ___

Answer 2

empirical (since you always have to reduce)

Question 3

How do you determine the empirical formula?

Answer 3

1. Base calculate on 100g sample of the compound. Determine # moles for each element in 100g. (Treat % composition as mass)
2. Divide each value of moles by the smallest value.
3. Multiply each number by an integer to obtain whole numbers.

Question 4

How do you determine the molecular formula?

Answer 4

1. Find the molar mass of the compound.
2. Divide the molecular mass by the mass of the empirical formula.
3. Multiply the empirical formula by this #.

Question 5

Law of definite proportions

Answer 5

A chemical compound always contains exactly the same proportion of elements by mass

Question 6

Excess reagent vs. limiting reagent

Answer 6

Excess reagent - the reactant that is present in more than the required amount for a complete reaction
Limiting reagent - the reactant that is completely consumed in a chemical reaction

Question 7

Stoichiometry is defined as…

Answer 7

the study of the relationship between quantities of reactants and products; mass and volume relationships in chemical changes