Calculations

Question 1

Formula for mass when given number of moles

Answer 1

mass = molar mass x moles

Question 2

Formula for number of moles given mass of sample

Answer 2

moles = (mass) / (molar mass)

Question 3

Units for mass, moles and molar mass

Answer 3

grams (g), moles (mol), gram/moles (g/mol)

Question 4

What is RAM?

Answer 4

Relative atomic mass - the mass of one mole of an element relative to 1/12th to mass of one mole of carbon-12.

Question 5

What does Avogadro’s number represent?

Answer 5

The number of particles in one mole.

Question 6

Formula for number of particles when given number of moles

Answer 6

Number of particles = Number of moles x Avogadro’s number

Question 7

Numerical value of Avogadro’s number

Answer 7

6.02x10^23 mol^(-1)

Question 8

Mass percentage equation

Answer 8

((total elative mass of element in molecule)/(Relative molecular mass))x100

Question 9

Mass percentage example

Answer 9

Step 1. write the word equation
Step 2. what information has been provided?
Step 3, find the difference between reactants and known product
Step 4. use the percentage equation

Question 10

Purity percentage

Answer 10

Purity percentage = ((mass of pure substance)/(mass of impure substance)) x 100

Question 11

Empirical formula

Answer 11

Shows the simplest whole number ratio of elements present in a compound.

Question 12

Molecular formula

Answer 12

Shows the actual number of atoms in a molecule.

Question 13

How to work out empirical formula

Answer 13

Step 1. note element and mass
Step 2. divide each by atomic mass
Step 3. divide each answer by the smallest number
Step 4. Not a whole number? Multiple the ratio by 2, 3 or 4
Step 5. This is the ratio for the formula - write formula

Question 14

How to work out molecular formula

Answer 14

Step 1. find the molar mass of the empirical formula
Step 2. divide the molecular formula molar mass by the empirical formula molar mass
Step 3. use this number to multiply the empirical formula ratio

Question 15

Definition of molar volume of gas

Answer 15

1 mole of any gas at room temperature (25oC) and room pressure (1 atm) occupies a volume of 24.8L

Question 16

Formula for volume of gas given number of moles

Answer 16

Volume = molar volume of gas x number of moles

Question 17

Volume

Answer 17

Space filled, unit of L (liters)

Question 18

Molar volume of gas at room temperature and pressure (SLC)

Answer 18

24.8 L/mol

Question 19

Theoretical yield

Answer 19

Maximum amount of product we could get from a given amount of reactants if we had no side reactions and all reactants collided successfully.

Question 20

Actual yield

Answer 20

Amount of product we get in a reaction.

Question 21

Percentage yield

Answer 21

Percentage yield = ((actual yield)/(theoretical yield))x100

Question 22

Molar concentration definition

Answer 22

Number of moles of solute per volume solution

Question 23

Equation for molarity (molar concentration) given number of moles of solute and volume of solution.

Answer 23

Molar concentration = (number of moles of solute)/(volume of solution)

Question 24

Titration

Answer 24

Titration is a technique to find the concentration of a solution.

Question 25

How to titrate accurately

Answer 25

Rinse to flush out potential contaminants.
The burette scale includes the volume of liquid in and below the tap.
Use clean flask to reduce contamination.
Pipette scale accounts for “last drop” to be left in pipette.
Need enough indicator to see a colour change, but not so much as to change the volume of the solution substantially.
White tile allows the colour change to be seen more clearly.
We can see where the level is between markings but cannot justify a precision more that 0.05 mL.
The trial run will give us an idea of when to go drop-by-drop. We go drop-by-drop to avoid over-shooting.
We repeat to get concordant results for reliability and to calculate the mean average for greater accuracy.

Question 26

Standard solution

Answer 26

Solution of accurately known concentration.

Question 27

Aliquot

Answer 27

Accurately known volume of the reagent solution with unknown concentration.

Question 28

Indicator

Answer 28

Gives visual indication of when reaction is complete.

Question 29

Equivalence point

Answer 29

The equivalence point marks the stage at which the reaction is complete, when neither reagent is in excess.

Question 30

End point

Answer 30

The end point is not the same as the equivalence point, but if care is taken the end point is the same as the equivalent point to the precision of the volume of one drop.