Moles

Question 1

How do you calculate Ar?

Answer 1

(the sum of isotopic masses x % abundance)/100

Question 2

How do you calculate the % mass of an element in a compound?

Answer 2

(Ar x no. atoms of that element)/Mr

Question 3

What is the mass of one mole of a substance equal to?

Answer 3

Its Mr

e.g 1 mole of carbon weighs 12g

Question 4

What are the 3 equations for moles?

Answer 4

Moles = mass/Mr
Moles = concentration (mol/dm^3) x volume (dm^3)
Moles = volume (dm^3)/24 (gases only)

Question 5

How else can concentration be given? What units? How do you convert it back?

Answer 5

can be given in g/dm^3

divide by Mr to convert back

Question 6

How do you work out the empirical formula from given masses? How do you convert this to molecular formula?

Answer 6

1) work out moles with the right equation
2) divide by the smallest to find whole number ratio
3) use ratio to write as a formula
( 4) divide the Mr that you’re given by the empirical mass you just got and then times it by the empirical formula
e.g. C2H6 e.m. = 30 given Mr = 90 90/30 = 3 therefore C2H6 => C6H18

Question 7

How do you work out the ratio in hydrated salts?

Answer 7

1) list the salt and the water
2) write the given masses for them
3) calculate the moles
4) divide by the smallest for whole no. ratio
5) write down what x/n stands for

Question 8

How do you work out the mass of something in an equation when you have the moles of something else?

Answer 8

1) work out moles of substance you are given mass of
2) divide by the number in front of the chemical whose moles you just worked out
3) times by the number in front of the compound you want to make
4) work out the mass of that many moles of that substance

Question 9

How do you work out the concentration of ions?

Answer 9

conc x vol x no. ions a compound holds (e.g. Na2CO3 has 2 Na+ ion but NaCl has 1 Na+ ion

Question 10

What is the predicted/theoretical yield?

Answer 10

the amount of product you expect to make from mole calculations

Question 11

What is the actual yield?

Answer 11

the amount of product you actually make

Question 12

How do you work out percentage yield?

Answer 12

(actual/predicted) x 100

Question 13

What are the reasons why you do not always achieve the theoretical yield?

Answer 13

• reaction doesn’t go to completion
• impurities in the reactants
• side reactions

Question 14

How do you get to constant mass?

Answer 14

Heat and weigh and heat and reweigh etc until the mass doesn’t change

Question 15

Why is it necessary to heat to constant mass?

Answer 15

to ensure all the water is removed

Question 16

Describe the practical to find to formula of copper oxide (7 steps)

Answer 16

1) weigh combustion tube and porcelain boat
2) put spatula of Copper Oxide in the boat and reweigh
3) flush gas through combustion tube for 10 secs before lighting bunsen burner underneath and excess gas at the hole
4) heat until all copper oxide => (pink) copper
5) turn off the heat but leave gas going thru tube til cooled (so cant react with oxygen)
6) weigh tube and boat and copper
7) find mass of copper and oxygen and work out moles and ratio

Question 17

Describe the practical to find the formula of MgO (6 steps)

Answer 17

1) weigh empty crucible and lid
2) clean Mg w sandpaper and coil
3) weigh crucible + lid + Mg
4) heat crucible with lid slightly off, gently for 1 min then strong
5) let crucible cool then weigh lid & contents
6) work out masses and moles of oxygen and Mg and find ratio and formula

Question 18

Describe the experiment to find the water of crystallisation of Copper Sulphate (4 steps)

Answer 18

1) weigh empty crucible, and then with 2/3 spatulas of blue copper sulphate
2) heat so the powder goes white (anhydrous) but not oxidised
3) let cool then reweigh
4) work out masses and moles of copper sulphate and water and the ratio