Topic 13 - Moles 1

Question 1

Define moles

Answer 1

They represent the amount of a substance

Question 2

Define empirical formula

Answer 2

The simplest mole ratio of the elements

Question 3

Describe molecular formula

Answer 3

The actual number of molecules of the elements

Question 4

How do you find the molecular formula

Answer 4

Divide the RFM of the substance by the RFM of the empirical formula

Question 5

Define water of crystallisation

Answer 5

Water molecules that are trapped inside salt crystals

Question 6

How do you calculate water of crystallisation ?

Answer 6

Find the moles of each substance and compare them

Question 7

What do balancing numbers in equations show?

Answer 7

The number of moles reacting and being produced

Question 8

Define percentage yield

Answer 8

Measures how much of a product is produced compared to the theoretical maximum

Question 9

How do you calculate percentage yield

Answer 9

Experiment
————— X 100
Calculation

Question 10

Describe empirical formula combustion practical

Answer 10

1) heat a crucible until its red hot (to make sure it’s clean and there are no traces of oil or water from a previous experiment)
2) leave the crucible to cool, then weigh it, along with it’s lid
3) add a specified mass of magnesium ribbon to the crucible.
4) heat the crucible with a lid on it to stop any solid escaping, but with a gap to allow oxygen through
5) heat until the magnesium ribbon has turned white (around 10 mins)
6) allow the crucible to cool and reweigh the crucible with the lid and it’s contents. The mass of the magnesium is this minus the mass of the crucible and lid

Question 11

Describe the empirical formula reduction practical

Answer 11

1) place a rubber bung (with a hole through the middle) into a test tube with a small hole in the end, and weigh them using balance.
2) take the bung out of the tube and spread a small amount of copper(II)oxide in the middle of the tube.
3) re-insert the bung and weigh the test tube again. Set up the experiment with the tube horizontally clamped above a Bunsen burner
4) expel the air from the test tube by gently turning on the gas. After 5 seconds, light the gas by holding a burning splint next to the hole at the end of the test tube. You can control the size of the flame by changing the amount of gas that flows through the test tube.
5) heat the copper(II) oxide for about 10 minutes (or until the solid changes from black to brownish-pink)
6) turn off the Bunsen burner and leave the test tube to cool
7) once the tube has cooled, turn off the gas and weigh the test tube with the bung and it’s contents.