Topic 1.2- quantitative chemistry

Question 1

how do you calculate relative formula masses from relative atomic masses?

Answer 1

• find the Ar for each element (top number of each element in the periodic table)
• add the Ar of each element together
• if the formula contains a small (subscript) number, it mean we must multiply the Ar by that number
• if the formula contains a bracket and a small (subscript) number it means we must times the Ar for the elements in the bracket by that number

Question 2

what is a mole?

Answer 2

a unit of amount in chemistry

Question 3

what are moles in a compound?

Answer 3

the mass in grams of one mole in a compound is equal to the molecular weight of the compound in atomic mass units eg. one mole of water H2O 1x2+16=18

Question 4

what is the calculation for moles?

Answer 4

http://3.bp.blogspot.com/-7tY4-HpJqx0/U-yvUpxnqfI/AAAAAAAAAEE/ul5cRbqydsc/s1600/mass_equals_moles_multiplied_by_ar_mr.png

Question 5

what is empirical formula?

Answer 5

the empirical formula shows the simplest ratio between atoms/ions in a compound

Question 6

how do you calculate empirical formula?

Answer 6

1. put the symbols for each element involved at the top of the page
2. underneath, write down the masses of each element combining
3. divide by their relative atomic masses (Ar)
4. divide all the numbers by the smallest of these numbers to give a whole number ratio
5. use this to give the empirical formula

Question 7

how do you calculate the molecular formula?

Answer 7

if you know the empirical formula and the relative formula mass you can work out the molecular formula of a compound

1. calculate the relative mass of the empirical formula
2. find out the number of times the relative mass of the empirical formula goes into the Mr of the compound
3. this tells us how many times bigger the molecule formula is compared to the empirical formula

Question 8

how do you calculate reacting masses?

Answer 8

calculate the mass of magnesium oxide that can be made by completely burning 6g of magnesium in oxygen. Equation for reaction: 2Mg + O₂—-> 2MgO

1. calculate for the amount, in moles, of magnesium reacted. Ar of Mg is 24; amount of magnesium= (6/24=0.025mol)
2. calculate the amount of magnesium oxide formed. The equation tells us that 2mol of Mg for 2mol of mgO, hence the amount of MgO formed is the same as the amount of Mg reacted (0.25)
3. calculate the mass of MgO formed. Mr of MgO+(24+16)=40. mass of magnesium oxide= (0.25x40)=10g

Question 9

what is concentration?

Answer 9

a measurement of the amount of substance per unit volume

Question 10

what is concentration measure in?

Answer 10

mol/dm3

Question 11

what is the equation for concentration?

Answer 11

concentration (mol/dm3)= amount(mol)/volumed(dm3)

Question 12

what is the equation for moles in terms of concentration?

Answer 12

amount(mol)=concentration(mol/dm3) x volume

Question 13

how do you convert cm into dm?

Answer 13

divide by 1000

Question 14

what is in the molar calculation triangle?

Answer 14

http://www.tutormyself.com/wp-content/uploads/img_566f06af5eb85.png

Question 15

how can the formula of simple compounds be found?

Answer 15

experimentally by reacting a metal with oxygen and recording the mass changes