Quantitative Chemistry

Question 1

how to find the volume of the gas?(moles)

Answer 1

(dm cubed)volume of gas = moles x 24

Question 2

how do you find the relative formula mass (Mr)?

Answer 2

Add together the relative atomic masses of all the atoms in that compounds molecular formula

Question 3

how to calculate the percentage mass of a particular element in a compound?

Answer 3

Ar of the element you trying to find out x number of that element in compound / Mr of the compound x 100 to turn into an percentage.

Question 4

How to find Moles?

Answer 4

Moles = mass / Mr

Question 5

Calculating Mass in Reactions example question.

Answer 5

‘Calculate the mass of sodium oxide (Na2O) that can be made by completely burning 5.0g of sodium (Na) in oxygen. Give your answer to 3 significant figures.’

1. When you see a question like this, think of there being a known substance and an unknown substance:

2.The known substance is the one they’ve told you the mass of - in this case the sodium.

3.The unknown substance is the one you’re trying to find the mass of - in this case the sodium oxide.

There are 4 steps to follow for questions like this:
1Write out the balanced equation

2Find the moles of the known substance (moles = mass ÷ Mr)

3Find the moles of the unknown substance (using the molar ratio from the balanced equation)

4Find the mass of the unknown substance (mass = moles x Mr)

1.
Write out the balanced equation

4Na + O2 ➔ 2Na2O
2.Find the moles of the known substance.

You’re told there are 5 g of sodium (Na), so find how many moles that is 5/23 = 0.217

3.
Find the moles of the unknown substance.

The unknown substance is sodium oxide (Na2O), so use the molar ratio to find the moles of Na2O

4Na + O2 ➔ 2Na2O

The molar ratio between 4Na and 2Na2O is 4:2 which is the same as 2:1

So divide the moles of Na by 2 to find the moles of Na2O
0.217 ÷ 2 = 0.1085 moles of Na2O
Find the mass of the Na2O (using the moles you just found)

mass of Na2O = moles x Mr

Mr of Na2O = (23x2) + (16) = 62

mass of Na2O = 0.1085 x 62

mass of Na2O = 6.727 g

give the answer to 3 significant figures

mass of Na2O = 6.73 g

Question 6

What is the conservation of mass?

Answer 6

no atom can be created or destroyed in a reaction the mass is always the same unless the reactants turn into gas and floats away so the end mass is lower.

Question 7

how to find the concentration(g/dm cubed) of a solution?

Answer 7

concentration = mass / volume

Question 8

how to find the concentration(mol/dm cubed) of a solution?

Answer 8

concentration = moles / volume

Question 9

how to find the Ar (atomic mass number)

Answer 9

(%abundance isotope 1 x Ar isotope 1)+(%abundance isotope 2 x Ar isotope 2) /100

Question 10

converting concentration?

Answer 10

concentration (mol/dm cubed) = concentration(g/dm cubed) /Mr

Question 11

percentage yield?

Answer 11

%yield= actual yield/ theoretical yield x 100%

Question 12

Atom economy?

Answer 12

atom economy= Mr of desired products/total Mr of all products x 100

Question 13

how to find energy change of reaction( used in finding bond energies)?

Answer 13

Energy change= total bonds broken-total bonds made

Question 14

how to find mean rate of a reaction ?

Answer 14

quantity of reactants used up(or products made)/time

Question 15

Rf value (chromatography)?

Answer 15

distance traveled by substance (to middle of spot)/distance traveled by solvent