Calculations

Question 1

What are moles

Answer 1

= amount of any substance that contains the same number of units as the number of atoms in exactly 12 grams of carbon 12. 1 mole of any substance = 6.022 x 10^23 units of substance

Question 2

What is the mass equation

Answer 2

m = M x n

-> m: mass (grams), M: molar mass (gmol^-1), n: moles (mol)

Question 3

What is Avogadro’s constant

Answer 3

6.022 x 10^23

Question 4

How to calculate molar mass

Answer 4

-> add relative atomic mass (RAM) of atoms in compound/molecule

Question 5

Avogadro’s Number + equation

Answer 5

-> can be used as a constant when calculating number of moles or number of particles
= 6.02 x10^23
- Equation : N= n x N^A
where N = number of particles (atoms, molecules or compounds) / n = number of moles / N^A = Avogadro’s constant (6.02 x 10^23)

Question 6

Percentage by mass equation

Answer 6

-> mass percent composition describes relative quantities of elements in a chemical compound

= (mass of element / mass of compound) x 100

Question 7

Purity percentage

Answer 7

equation = (mass of pure substance / mass of impure substance) x 100

Question 8

What is the empirical formula

Answer 8

• > shows the simplest whole number ratio of elements present in a compound
  e. g Hydrogen peroxide (H^2O^2) = 2:2, therefore empirical formula = HO

Question 9

What is the molecular formula

Answer 9

• > shows the actual number of atoms in a molecule

e. g Hydrogen peroxide = H^2O^2

Question 10

How to work out molar volume of gas

Answer 10

• > Volume of gas (V) is given by the volume taken up by one mole of gas under given temperature + pressure (V^m), multiplied by the number of moles (n) of gas in the sample.
• > 1 mole of gas @ room temp (25c) + room temp (100kpa) [= standard lab conditions] occupies a volume of 24.8L

V = V^m x n

Question 11

Calculating theoretical yield

Answer 11

Percent yield = (actual yield) / (theoretical yield) x 100

• > Actual yield = amount of product we get in reaction
• > Theoretical yield = found by using relative formula masses with equation 4 reaction 2 calculate max. amount of product we could get from a given amount of reactants.

Question 12

Percent yield

Answer 12

Percent yield = (actual yield) / (theoretical yield) x 100

• 4 reaction to occur, reactants must collide with sufficient energy in correct orientation
• > Reactions in practice are nvr complete h/r 2 make products with min amount of reactants, we are interested in percentage yield = amount of product obtained compared with theoretical maximum (out of 100)

Question 13

Mole concentration

Answer 13

• Mass concentration can be written as mass per litre -> gL^-1
• H/r more useful 2 know molar concentration b/c most calculations involve knowing how many moles we have
• Molar concentration can be written as moles per litre -> molL^-1
• 1 mol L^-1 = 1M, capital M = a unit of concentration e.g 1.5M HCl per litre

Question 14

Molar concentration equation

Answer 14

n = CV

Where: c = concentration / n = number of moles / v = volume

Question 15

What’s a titration

Answer 15

= a technique to find the concentration of a solution, useful in quality control situations
-> one of the solutions must be a standard solution of accurately known concentration

Question 16

What is an aliquot

Answer 16

= an accurately known volume of a reagent solution

Question 17

What is an indicator

Answer 17

= gives a visual indication that reaction is complete

Question 18

What is the end point in a titration

Answer 18

-> indicator colour change marks the end point of the titration

Question 19

What is the equivalent point

Answer 19

= marks the stage at which the reaction is complete, when neither agent is in excess

Question 20

Titration method

Answer 20

1) A burette containing a solution of one of the reagents
2) A flask containing an accurately known volume of the other reagent solution ( called an aliquot), added using a pipette
3) Some way 2 indicate when the reaction is complete. In cases we consider this will be adding an indicator to the contents of the flask that gives a visual indication
4) The emphasis is on accuracy, getting as close as possible to the “true value”