Flashcards in Topic 1.2- quantitative chemistry Deck (15)

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1

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

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- 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

2

## what is a mole?

### a unit of amount in chemistry

3

## what are moles in a compound?

### 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

4

## what is the calculation for moles?

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

5

## what is empirical formula?

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

6

## how do you calculate empirical formula?

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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

7

## how do you calculate the molecular formula?

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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

8

## how do you calculate reacting masses?

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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

9

## what is concentration?

### a measurement of the amount of substance per unit volume

10

## what is concentration measure in?

### mol/dm3

11

## what is the equation for concentration?

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

12

## what is the equation for moles in terms of concentration?

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

13

## how do you convert cm into dm?

### divide by 1000

14

## what is in the molar calculation triangle?

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

15