Quantitative - PowerPoint Presentation

Slide1

Quantitative Chemistry

Calculations

in Chemistry:

part

2Slide2

Compound Formation

A

particular

compound

always

contains

the

same

elements

.

Independent

of

how

you

make

the

compound

the

elements

are

always

present

in

the

same

proportion

by

mass

.

These

proportions

cannot

be

changed

.

Magnesium

oxide

always

contains

60% Mg

and

40% O

by

mass

.Slide3

Reacting amounts

of

substance

Relative

formula

masses

can

also

be

used

to

calculate

the

amounts

of

compounds

reacted

together

or

produced

in

reactions

.

If

0.24g

of

Mg

react

with

0.16g

of

O

to

produce

0.40g

of

MgO

..........

How

much

MgO

will

be

produced

by

burning

12g

of

Mg?

0.24g Mg

produces

0.40g

of

MgO

so 1g

of

Mg

produces

0.40/0.24

g

of

MgO

= 1.67

g

of

MgO

so 12g

of

Mg

produces

12 x 1.67

g

MgO

= 20g

of

MgOSlide4

Important

Calculations

of

quantities

like

these

are

a

very

important

part

of

chemistry

.

There

is

a

great

deal

of

information

stored

in

the

formulae

and

the

equation

.Slide5
Slide6

Activity

Question

3 on

page

181

Extension: Read

through

pg

178 – 179 (

compound

formation

and

chamical

formulea

.

Answer

question

4 on

page

181Slide7

The Mole

When

carrying

out an

experiment

a

chemist

can

not

weigh

out

single

atoms or molecules.A counting unit was found.The standard unit of a substance is the relative atomic mass in grams.This unit is called 1 mole (1 mol)Carbon Ar = 121 mole of carbon is 12g

In a

similar

way

banks

weigh

coins

because

they

know

how

much

one

coin

weighsSlide8

Avogadro‘s Constant (L)

1

mole

of

any

substance

contains

the

same

number of atoms.6.02 x 1023 atoms per mole6.02 x 1023 coke cans stacked together would cover the surface of the Earth to a depth of 200 miles. Slide9

Calcualtions involving

the

Mole

How

to

calculate

the

molar

mass

.Write the formula for ethanol C2H5OHCalcualte Mr = (2x12) + (5 x 1) + 16 + 1 = 46The molar mass of ethanol is 46g/molSlide10

Calculations

For

any

given

mass

of

a

substance

you

can calculate the moles present. MassNumber Mr Of MolesSlide11

Example

How

many

moles

are

there

in 60g

of

S

odium Hydroxide?Mr of NaOH = 23 + 16 + 1 = 40Molar mass of NaOH = 40g /molNumber of moles = mass / molar mass = 60 / 40 = 1.5 molsSlide12

Activity

Worksheet

6.2Slide13

Working out the Empirical

Formula

We

can

work

out

the

chemical

formula

from experimental data.Magnesium reacts with oxygen to form magnesium oxide. In an experiment if we know that 0.24g of Mg reacts with 0.16g of O2 we can determine the formula.MgOMass combinedo.24 g0.16gMolar mass

24 g/

mol

16 g/

mol

Number

of

moles

0.01

mol

0.01

mol

Simplest

ratio

1

1

formula

MgOSlide14

Activity: Calculate

the

empirical

formula

for

silicon

oxide

Si

OMass47g53gMolar massNumber of molesSimplest rationFormulaComplete questions on page 185Slide15

A Chemical

Footbridge

:

calculating

reacting

amounts

What

mass

of

aluminium oxide is produced when 9.2g of aluminuim metal react with oxygen?Balanced equation:4Al + 3O2 2Al2O3 Ratio 4:2Convert 9.2 g of Al into moles.Number of moles = mass/ molar mass = 9.2 / 27 = 0.34 molUse the ratio from the equation to work out how many

moles

of

Al

2

O

3

are

produced

.

4

mol

of

Al

produce

2

mol

of Al2O3So 0.34 mol of Al produce 0.17 mol of Al2O3Slide16

A Chemical Footbridge:

calculating

reacting

amounts

…..

continued

Work out

the

mass

of Al2O3Mr = 102Mass = molar mass x number of moles = 102 x 0.17 = 17.3 gSlide17

Concentrations of Solutions

Two

types

of

concentration

Mass

concentration

- g/dm³

Molar

concentration

- mol / dm³1dm³ = 1000 cm³ = 10 cm x 10 cm x 10cm = 1 LitreConcetration = mass of solute volume of solutionSlide18

Concentration

A 1

mol

/dm³

solution

of

sodium

chloride

contains

58.5g

of NaCl (1 mol) dissolved in water and made up to a final volume of 1 dm³ or 1 litre.Slide19

Concentration calculations

How

many

moles

of

suger

are

there

in 500 cm³ of a 3.0 mol/dm³ sugar solution?Number of moles = concentration x volume 1000 = 3.0/ 1000 x 500 = 1.5 molSlide20

Example

Calculate

the

concentration

of

a

solution

of

sodium

hydrioxide , NaOH, that contains 10g of NaOH in a final volume of 250 cm³ (= 0.25 dm³)Calculate number of molesMr = 23 + 16 + 1 = 40Number of moles = mass/ Mr = 10/40 = 0.25 molFind the concentrationNumber of moles = concentration x volumeConcentration = number of moles / volume = 0.25 / 0.25 = 1 mol /dm³ Slide21

Activity

Read

pages

192

and

193