Moles 2 - molecular and empirical formula

Question 1

What is the molecular formula?

Answer 1

shows the number of atoms of each element in one molecule of the substnace
it tells you how many atoms of each element are in one molecule
They do not tell you how they are arranged

Question 2

What is the empirical formula?

Answer 2

shows the simplest whole number ratio of atoms of each element present in a substance

Question 3

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

Answer 3

mass of the element in the compound/ total mass of compound
x100

Question 4

How do you work out the empirical formula?

Answer 4

1. copy down the mass/ %mass for each element
2. divide each elements mass by their Ar (moles)
3. divide all by the smallest answer found in step 2

Question 5

How do you work out the molecular formula from the empirical formula?

Answer 5

mr molecular / mr empirical = multiplier for molecular

Question 6

What is a hydrated salt?

Answer 6

when some ionic compounds are crystallised from an aquwous solution, the crystals contain water molecules which are locked in place around the ions.

Question 7

How do you find an element of the salt?

Answer 7

1. find water mass
2. moles of water
3. find moles of compound using mole ratios
4. calculate Mr of compund
5. calculate Ar of element

= M(NO3)2 .4H2O
finding M

Question 8

How do you find the formula of a hydrated salt?

Answer 8

1. find moles of anhydrous salt
2. find mass of water
3. calculate moles of water
4. find ratio of water to salt - mol water / mol salt

= CuSO4 . XH2O
finding X

Question 9

How do you construct an equation for removing the water of crystallisation from a hydrated salt?

Answer 9

salt . XH20 (s) —— salt (s) + XH2O (g)

Question 10

What is the theoretical yeild?

Answer 10

the maximum possible amount of product

Question 11

Why is it diffucult to acheive the theoretical yeild?

Answer 11

the reaction may have not gone to completion
other reactions may have taken place alongside the main reaction
purification of the product msy result in loss of some product

Question 12

What is the actual yeild?

Answer 12

the amount of product that is actually produced in the reaction

Question 13

How do you work out % yeild?

Answer 13

actual / theoretical x 100

Question 14

What is the limiting reagent?

Answer 14

the reacntant that is not in excess will be completely used up first and stops the reaction

Question 15

What is the atom economy?

Answer 15

the atom economy of a reaction is a measure of how well atoms have been utilised

Question 16

What are the advantages of reactions with high atom economies?

Answer 16

they produce a large proportion of desired products and few unwanted waste products
they are important for sustaintability as they make the best use of natural resources

Question 17

How do you calculate the atom economy?

Answer 17

sum of Mr of desired products / sum of Mr of all products x 100