3.1.2: Amount of Substances

Question 1

What is the definition of Relative Molecular mass?

Answer 1

Relative molecular mass is the ratio of the average mass of one molecule of an element or compound to 1/12 of the mass of an atom of carbon-12.

Question 2

What is Avogadro’s constant?

Answer 2

6.02×10²³mol-¹

Question 3

How to find no. of moles?

mass/Mr

Answer 3

no. moles= mass (g)÷Mr (gmol-¹)

Question 4

How to find reacting masses?

from a BALANCED equation

Answer 4

1. find moles in X (given mass)
2. find ratio of moles in X:? proportional with X moles
3. use no.= m/Mr to find mass of?

Question 5

Equation for mass

Mr/a.c.

Answer 5

mass=Mr (gmol-¹)÷ avogadro’s number

in grams

Question 6

How to find no. moles?

c/v

Answer 6

no. moles= concentration (gdm-³)× volume (dm³)

Question 7

Ideal gas equation

Answer 7

pV=nRT
p-ressure (Pa)
V-olume (m³)
n-o. moles (mol)
R-gas constant (8.31Jmol-¹K-¹)
T-emperature (K/C+273)

Question 8

Ideal gas unit conversions

volume and pressure

Answer 8

cm³–>dm³ =×10³ dm³–>m³=10³

Pa–>kPa =×10³ kPa–>MPa=10³

Question 9

Empirical formula

Answer 9

• divide % quantity by the relative atomic mass/Ar
• divide results of each element by smallest value
• round to whole number
• find ratio of elements
• make empirical formula

if you’re given a mass instead of a %, find % of each element

Question 10

Molecular formula

Answer 10

• find empirical formula
• find Mr of empirical formula
• take molecular mass given
• do molecular mass÷ empirical formula Mr
• multiply answer by element no. of each element to get molecular formula

Question 11

Percentage Yield

shows how much product actually forms comosred to theoretical amount

Answer 11

% yield= product obtained ÷ theoretical product amount

×100

this can be in mass (g) or moles (mol)

Question 12

Atom economy

considers amount of waste product produced

Answer 12

a.e.= m of desired product÷ m of all reactants/products

EQUATION NEEDS TO BE BALANCED /CAN BE IN MASS OR MR

Question 13

Titration to find mass of acid (NaOH)

finding out concentration of an unknown acid/alkali with a given conc.

Answer 13

• use concordant (within 0.1 cm³ of each other) titre volume to find mean volume
• use n=cv to find n. or acid
• use ratio to find n. of NaOH
• Do volume of solution÷volume of sample
• multiply answer by n. of NaOH
• use n= m/Mr to find mass

Question 14

What’s the difference between hydrated and anyhydrous salts?

Answer 14

hydrated- contains water of crystallisation
anyhydrous- lacks water of crystallisation
- the water of crystallisation can be driven off by heating salt crystals to leave anyhydrous salts
- all water is lost until mass is constant

Question 15

Find how much water/ how hydrated an element is

e.g. SnCl².XH²O

Answer 15

• find empirical formula
• find Mr of each element
• use given mass
• use n= m/Mr to find the no. moles
• find X