Chemistry - moles 1 and 2

what are moles

the amount of substance, the unit is moles or mol

what is the relative atomic mass

the mass found on the periodic table for an element

what is the relative formula mass

the sum of all the atomic masses of atoms represented in a chemical formula

what is the relative molecular mass

a term for formula that can be used with molecules

what is the empirical formula

shows the simplest whole number ratio of the atoms of different elements present in a compound

what is the limiting reagent

reactant that there are fewer moles than required by ratio given in equation

what is the reagent in excess

reactant that there are more moles than required by ratio in the equation

what is the reagent

reactant in chemical reaction

how to work out the percentage composition

Ar(of element in compound) x no. of atoms of the element in the compound/Mr of compound x 100

how to find the percentage composition of water

Mr of water x no. of water molecules in the compound/ Mr of compound x 100

what is a score of moles worth

20 units

what is Avogadro’s constant

6 × 10^23

how to find the moles of a substance

from the mass in grams and the relative formula mass

how to find the moles of an element

mass (g)/ Ar (g/mol)

how to find the moles of a compound

mass (g)/ Mr (g/mol)

how can the empirical formula be calculated

using either masses or percentage composition by mass

experiment for magnesium oxide

1. weigh a clean dry crucible

2. add a piece of magnesium ribbon and reweigh it with the lid on

3. using a pipe-clay triangle and a lid, strongly heat the magnesium

4. reweigh the crucible and lid

5. repeat steps 3-4 times until the mass remains constant

what could be some possible sources of error in the experiment of determining the empirical formula of magnesium oxide

not heated to constant mass or reaction incomplete, some MgO lost during the experiment, not weighing with the lid on consistently

experiment of copper oxide

1. weigh the empty reduction tube

2. add two spatulas of the copper oxide into the tube and reweigh.

3. turn on the gas and heat using a Bunsen burner until reaction is complete

4. reweigh the tube and its content.

5. repeat for the other copper oxide

what were the droplets of liquid seen at the end of the reduction tube before the apparatus became extremely hot in the copper oxide experiment

water

what did each copper oxide sample look like before and after reduction?

copper (ll) oxide - fine black powder at start, pink-brown metal at end

copper (l) oxide - fine red powder at start, pink-brown metal at end

why is methane passed over the product whilst it cools in the copper oxide experiment ?

to prevent the copper from reacting with oxygen in the air that would otherwise be sucked back into the tube

how to check that the pink-brown solid at the end of the copper oxide experiment is a metal ?

set up a circuit with a sample connected in the series and see if the light bulb lights up

have you learn the picture

yes

experiment for reduction of tungsten oxide WO3

1. record the mass of a weighing boat

2. add tungsten oxide and record the mass again

3. heat the weighing boat and tungsten oxide strongly for two minutes, then allow to cool

4. record the mass of the weighing boat and its content

have you learnt the picture

yes

what could be an addition to the method to check that the tungsten oxide has been completely reduced

heat to constant mass

safety precautions for tungsten oxide experiment

lab coat, eye protection, use safety screen

what is molecular formula mass

number of atoms x empirical formula mass