Calculations

Question 1

Calculating mass

Answer 1

mass = molar mass x moles
m = M x n

Question 2

Calculating moles

Answer 2

moles = mass/molar mass
n = m/M

Question 3

Units for mass, moles and molar mass

Answer 3

mass = g
moles = mol
molar mass = gmol^-1 or g/mol

Question 4

RAM

Answer 4

Relative Atomic Mass
The mass of an atom is so small, it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass.

Question 5

Avogadro’s number

Answer 5

6.022 x 10^23 is Avogadro’s number (N(subscript)A).

1 mole of any substance = 6.022 x 10^23 units of that substance.

Question 6

Moles

Answer 6

A mole is the amount of any substance that contains the same number of units as the number of atoms in exactly 12 grams of carbon-12.

Question 7

Molar Mass

Answer 7

Molar mass is the mass in grams of one mole of a substance.

Question 8

N (particles)

Answer 8

n x N(subscript)A

Question 9

Avogadro’s Number

Answer 9

6.02 x 10^23 = N(subscript)A

Question 10

Mass Percentage equation

Answer 10

Percentage by mass = molar mass of element/molar mass of compound x 100

Question 11

Mass percentage example

Answer 11

What is the mass percentage of Sulfur in Sulfur dioxide?

Molar mass (SO2) = 64.1 g/mol
Molar mass (S) = 32.1

Percentage by mass = 32.1/64.1 x 100 = 50.1%

Question 12

Purity percentage

Answer 12

Mass of pure substance/ mass of impure substance x 100

Question 13

Definition of empirical formula

Answer 13

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

Question 14

Definition of molecular formula

Answer 14

Shows the actual number of atoms in a molecule.

Question 15

How to work out empirical formula (e.g. 10g of C and 1.6g of H)

Answer 15

1. rewrite mass of each element (same with percentage)
2. divide mass of each element by RAM
3. divide each answer by smallest number
4. whole numbers are now the ratio

e.g.
C = 10g H = 1.6g
10/12 = 0.83 1.6/1 = 1.6
0.83/0.83 1.6/0.83
= 1 = 2
–> CH2

Question 16

How to work out molecular formula (e.g. EF: CH2 and molar mass = 84g/mol)

Answer 16

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

e.g. 
CH2 = 12 + 1 + 1 = 14g/mol
84/14 = 6
1:2 x 6 = 6:12
C6H12

Question 17

Molar Volume of a gas

Answer 17

V = Vm x n

Question 18

value of V

Answer 18

Volume in L

volume of a gas
V = Vm x n

Question 19

Molar volume value

Answer 19

Vm = 24.8

Question 20

Definition of molar volume of gas

Answer 20

“1 mole of any gas at room temperature (25*C) and room pressure (1 atm) occupies a volume of 24.8L”

Question 21

Quantity unit conversions

Answer 21

1000 ml = 1L = 1000cm3
1cm3 = 1 mL
24800 cm3 = 24800 mL = 24.8 L

Question 22

Theoretical Yield

Answer 22

Calculated mass (g) or volume (L).

Question 23

Actual Yield

Answer 23

Mass (g) or volume (L) actually produced in an experiment.

Question 24

Percent Yield

Answer 24

How much of the theoretical yield was actually produced.

Question 25

Molar concentration

Answer 25

moles per litre

mol/L, molL^-1, M = unit of concentration

Question 26

molar concentration formula

Answer 26

C = n/V
molar concentration = moles of solute/volume of solution

n = C x V
moles of solute = volume of solution x molar concentration

Question 27

Titration

Answer 27

A way to measure volumes precisely.

Method:

1. Place burette on the floor with the tap closed. Using a funnel carefully fill the burette with NaOH.
2. Measure out 20mL of HCl from a beaker using a glass pipette along with a pipette head and deliver this to a clean conical flask.
3. Add 3 or 4 drops of phenolphthalein indicator to the conical flask. This should stay colorless.
4. Place a white tile at the base of the burette stand and place the conical flask on the tile.
5. Record the initial reading from the burette to 2 decimal places.
6. Slowly deliver the NaOH into conical flask, drop by drop.
7. When a single drop causes the color to permanently remain, stop and record the final reading.
8. Repeat steps until at least 3 concordant titres are obtained.

Question 28

Features in method to improve accuracy

Answer 28

1. Potential contaminants removed
2. Flask is clean to reduce contamination.
3. Need enough indicator to see a color change, but not so much as to change volume of solution substantially.
4. White tile allows color change to be seen more clearly - we want persistent hint of pink.
5. Repeat to get concordant results for reliability and to calculate the mean average for greater accuracy.

Question 29

Standard Solution

Answer 29

Accurately known concentration.

Question 30

Aliquot

Question 31

Indicator

Answer 31

Solution added to give visual indication of the end point.

Question 32

Equivalence point

Answer 32

Marks the stage at which the reaction is complete, when neither reagent is in excess.

Question 33

End Point

Answer 33

The point at which the acid and alkali have reacted completely. The indicator color change marks the end point.