Formulae | Quiz 1

Question 1

Number of moles =

Answer 1

Actual mass (g) / molar mass

OR

of particles / Avogadro’s number

Question 2

Absolute uncertainty is expressed as:

Answer 2

+/- [uncertainty value]

Question 3

When adding or subtracting, uncertainty should be:

Answer 3

added together in the sum/difference

Question 4

When multiplying or dividing, uncertainty should be:

Answer 4

converted into percent uncertainty, and then added

Question 5

The formula for converting absolute uncertainty into percent uncertainty is:

Answer 5

absolute uncertainty / measurement value * 100

Question 6

The formula for determining experimental error is:

Answer 6

( |actual measurement - expected measurement| / expected measurement ) * 100

Question 7

The empirical formula of a compound, when given the # of grams of its components, can be determined by:

Answer 7

1. Converting mass into moles
2. Taking the mole ratio
3. Finding the whole number ratio

Question 8

The empirical formula of a compound, when given the percent composition of its components, can be determined by:

Answer 8

1. Assuming that you have 100g of the compound
2. Converting mass into moles
3. Taking the mole ratio
4. Finding the whole number ratio

Question 9

The molecular formula of a compound, when given the empirical formula, can be determined by:

Answer 9

1. Using the given molar mass of the molecular compound
2. Determining the molar mass of the empirical formula
3. Comparing (e.g. EF = 30, MM = 180)
4. Multiplying components as needed

Question 9

Percentage composition by mass of a given element within a compound can be determined by:

Answer 9

Dividing [mass of element in 1 mole of compound] / [molar mass of compound]

Question 10

Determine the percentage composition by mass of H2O.

Answer 10

%H = 2/18 = 11.1%
%O = 16/18 = 88.9%

Question 11

The Law of Definite Proportions was stated by whom? What does it mean?

Answer 11

1. Joseph Proust
2. That a compound will always have a fixed ratio of elements (H2O will always be 2:1)

Question 12

Explain how empirical formula can be determined via combustion?

Answer 12

1. Find mass of CO2 and H2O produced by combustion process
2. Find mass of carbon in CO2 and mass of hydrogen in H2O (mass * percent composition)
3. Determine number of moles of carbon and hydrogen
4. Take the mole ratio
5. Take the whole number ratio

Question 13

The relationship between uncertainty and experimental error is:

Answer 13

Experimental error - uncertainty = error% that can be attributed to systemic error. Uncertainty is based in precision, whereas experimental error is based in accuracy.

Question 14

Distinguish systemic error from random error:

Answer 14

Systemic error is caused by errors in the procedure and can be reduced by modifying the experiment. Random error is caused by errors in the equipment and cannot be reduced by modifying the experiment.