Amounts of Substance

Question 1

Avogadros Constant

Answer 1

equal to the number of atoms in 12g of C12 = 1 mole

Question 2

Calculating number of particles (element in a molecule)

Answer 2

number of particles= moles x avogadros constant

Question 3

Relative Atomic Mass (Ar) definition

Answer 3

average mass of one atom of element / 1/12th mass of a C12 atom

Question 4

Relative Molecular Mass (Mr) definition

Answer 4

average mass of one molecule / 1/12th mass of a C12 atom

Question 5

Moles equation

Answer 5

moles = mass / Mr or Ar

Question 6

Percentage yield equation

Answer 6

% yield = amount of specific product formed / theoretical max amount produced x 100

Question 7

How to calculate % yield

Answer 7

Reacts 2.0cm^3 propan-2-ol. obtains 0.954g propanone. calc % yield of propanone.
Density propan-2-ol = 0.786g cm^3

amount/theo x 100
mass= density x vol
( g mol mol g)

2 x 0.786 = 1.5272g (Mr=60)
1.5272 / 60 = 0.00262 (Mr=58)
0.00262 x 58 = 1.5196g

0.954 / 1.5196 x 100 = 62.8%

Question 8

Concentration equation

Answer 8

concentration = moles / volume

Mol/dm^3 → g/dm^3 (x Mr)
g/dm^3 → Mol/dm^3 ( /Mr)

Question 9

Percentage Purity equation

Answer 9

% purity = mass of pure / mass of impure x 100

Question 10

Titration

Answer 10

• fill burette with acid of known conc
• measure 25cm^3 of alkali standard solution
• add few drops of indicator to alkali
• slowly add acid into conical flask, swirling constantly. as soon as indicator changes colour, solution neutral so stop adding acid
• record vol of acid added
• repeat until you get concordant results (within 0.1cm^3)
• mean used to calc average titre
• average titre can be used to calc unknown conc

Question 11

Standard solution

Answer 11

1. weigh sample bottle containing solid on balance
2. transfer to beaker and reweigh sample bottle
3. record difference in mass
4. add distilled water
5. stir
6. until all solid dissolved
7. transfer to volumetric flask using funnel
8. with washings
9. make up to 250cm^3
10. shake

Question 12

How to reduce titre uncertainty?

Answer 12

use larger mass of solid, so larger titre needed

Question 13

Titration equations

Answer 13

• write equation
• calculate moles
• use ratio
• calculate concentration

Question 14

Percentage error equation

Answer 14

% E = +- error / amount measured x 100

Question 15

Ideal Gas equation

Answer 15

PV = nRT

p= pressure (Pa) KPa ⇢ Pa (x1000)
v= volume (m^3) cm⇢dm (/1000) dm⇢m (/1000)
n= moles of gas
r= molar gas constant (8.31J k-1 mol-1)
t= temperature (k) k= ℃ + 273

Question 16

Problems with ideal gas

Answer 16

some liquid not evaporated
so volume recorded is lower
Mr is greater than real Mr

Question 17

Empirical formula definition

Answer 17

simplest whole number ratio of atoms of each element

Question 18

Molecular formula definition

Answer 18

actual number of atoms of each element

Question 19

calculating empirical formula

Answer 19

• element
• mass
• /Ar
• / small value
• ratio

Question 20

calculating molecular formula

Answer 20

• find Mr (44)
• Mr of molecular (88) in Question
• 88 / 44 =2
• molecular = empirical x2

Question 21

Percentage atom economy

Answer 21

% atom economy = mass (Mr) of desired products / total mass reactants x 100