5. Formula and amounts (MOLE)

Question 1

What are amounts of substances measured in

Answer 1

Moles (mol)

Question 2

What is Avagadro’s constant?

Answer 2

The number of atoms, molecules or ions (particles) in one mole of a substance.
–> 6.02 x 10^23 mol-1

Question 3

State Avagadro’s law

Answer 3

The volume of a gas is directly proportional to the number of moles of gas, when the temperature and pressure are held constant.

Question 4

What is the molar mass of a substance

Answer 4

The molar mass is the mass per mole of a substance in g mol^-1

Question 5

How do you work out the molar mass of a substance

Answer 5

Add up the relative atomic masses of all the elements in the molecule
eg: CaCO3 - 40.1 + 12 + 16 + 16 + 16 = 100.1

Question 6

Give the formula to work out the number/ amount of moles in a substance

Answer 6

moles (mol) = mass (g) / molar mass (g mol^-1)

Question 7

How would you work out the number of particles or atoms in a substance from the number of moles it has

Answer 7

Moles ( mol) x avagadros constant

Question 8

Give the equation to work out the concentration of a substance using the number of moles it has

Answer 8

concentration ( mol dm^-3 ) = moles (mol) / volume (dm^3)

Question 9

How do you convert concentration in moldm -3 into gdm-3

Answer 9

Multiply the concentration by Mr

Question 10

What is the molecular formula

Answer 10

The actual number of atoms of each element within a compound

Question 11

What is the empirical formula

Answer 11

The simplest whole number ratio of the atoms of each element within a compound

Question 12

Describe how to work out the empirical formula of a compound from experimental results and from percentages

Answer 12

1) Write out the mass of each element
2) Write down molecular mass (Mr) of each element
3) From the information above work out the number of moles using mass/ Mr
4) Divide all the numbers by the smallest number of moles
5) This will have produced a ratio of each element in the compound

Question 13

what should you do if one of the ratios of the empirical formula ends in:
0.25
0.3
0.5
0.67

Answer 13

0.25 - times everything by 4
0.3 - times everything by 3
0.5 - times everything by 2
0.67 - times everything by 3

Question 14

Give the 3 steps to form an ionic equation

Answer 14

1) write a balanced symbol equation, complete with state symbols
2) Add the charges to all the aqueous substances eg: NaCl becomes Na+ Cl-
3) Cancel out all the spectator ions (ions that remain same before and after reaction)