Combined Chemistry - 5.3 Flashcards Preview

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Flashcards in Combined Chemistry - 5.3 Deck (23)
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1
Q

What does the law of the conservation of mass state?

A

No atoms are lost or made during a chemical reaction

2
Q

A piece of magnesium is added to acid. What would happen to the mass reading on the balance and why?

A

It would stay the same because there is a lid on the conical flask so no gas can escape.

3
Q

Count the number of elements in the following compounds:

a) CaO
b) Al2O3
c) H2SO4

A

a) CaO - 2 elements (Ca and O)
b) Al2O3 - 2 elements (Al and O)
c) H2SO4 - 3 elements (H, S and O)

4
Q

Count the number of atoms in the following compounds:

a) CaO
b) Al2O3
c) H2SO4

A

a) CaO - 2 atoms (1 x Ca and 1 x O)
b) Al2O3 - 5 atoms (2 x Al and 3 x O)
c) H2SO4 - 7 atoms (2 x H, 1 x S and 4 x O)

5
Q

Calculate the relative formula mass (Mr) of each of the compounds (a, b & c) below.

AR: Mg = 24; Cl = 35.5; Ca = 40; O = 16; Al = 27

a) CaO
b) MgCl2
c) Al2O3

*this is just an example, you need to be able to do this for any compound.

A

a) CaO: 40 + 16 = 56
b) MgCl2: 24 + (35.5x2) = 95
c) Al2O3: (27x 2) + (16 x 3) = 102

6
Q

A piece of magnesium is added to acid. What would happen to the mass reading on the balance and why?

A

It would decrease because there is no lid on the conical flask so gas can escape and therefore mass is ’lost’ from the flask.

7
Q

Calculate the uncertainty in the following sets of results

a) 1, 3, 6, 7, 10
b) 22, 25, 26, 25, 24

*this is just an example, you need to be able to do this for any set of data given.

A

Use the equation: uncertainty = range / 2

a) 1, 3, 6, 7, 10

Range = 10 – 1 = 9. Uncertainty = 9/2

Uncertainty = ± 4.5

b) 22, 25, 26, 25, 24

Range = 26 – 22 = 4. Uncertainty = 4/2

Uncertainty = ± 2

8
Q

Calculate the percentage mass of hydrogen in:

Ar: N = 14 ; H = 1 ; S = 32 ; O = 16.

a) NH3
b) H2SO4

Give your answer to the nearest whole number.

*this is just an example, you need to be able to do this for any compound when given the appropriate information.

A

% mass = (Ar x number of atoms of the element) / (Mr of compound) x 100

a) NH3 : (3x1) / 17 = 0.176
0. 176 x 100 = 18 %
b) H2SO4 : (2x1) / 98
0. 02 x 100 = 2 %

9
Q

Higher Q. How many molecules, atoms or ions are in one mole of a substance?

A

6.02 x 1023

10
Q

Higher Q. How many moles are in the following?

a) 46 g of CaO?
b) 50 g of O2?

A

Number of moles = (mass in g)/Mr

a) Mr of CaO = 40 + 16 = 56.

Number of moles = 46 g /56 = 0.82 moles

b) Mr of O2 =16

Number of moles = 50g / 32 = 1.6 moles

11
Q

Higher Q. Describe the equations below in words in terms of the number of moles of each substance reacting.

1) 2Mg + O2 → 2MgO
2) H2SO4 + 2NaOH → Na2SO4 + 2H2O

A

1) 2 moles of magnesium react with one mole of oxygen to produce two moles of magnesium oxide.
2) One mole of sulfuric acid reacts with two moles of sodium hydroxide to produce one mole of sodium sulfate and two moles of water.

12
Q

Higher Q. Calculate the mass of magnesium needed to produce 4.6 tonnes of magnesium oxide.

2Mg + O2 → 2MgO

*this is just an example, you need to be able to do this for any equation when given a mass of a reactant or product.

A

Step 1 – Make sure the equation is balanced - This equation is already balanced.

Step 2 – Calculate the Mr of 2Mg and the Mr of 2MgO.

Mr of 2Mg = 48, Mr of 2MgO =80

Step 3 – You now know 48 tonnes of magnesium would produce 80 tonnes of magnesium oxide.

Step 4 – Find how much magnesium you would need to produce 1 tonne of magnesium oxide: (48/80) magnesium and (80/80) magnesium oxide.

Therefore 0.6 tonnes of magnesium would produce 1 tonne of magnesium oxide.

Step 5 – Multiply both sides by 4.6 to work out how much magnesium would be needed to make 4.6 tonnes of magnesium oxide.

(0.6 x 4.6) = 2.76 tonnes

13
Q

Higher Q. Calculate the mass of CO2 produced when 50 g of CaCO3 is thermally decomposed.

CaCO3 → CaO + CO2

*this is just an example, you need to be able to do this for any equation when given a mass of a reactant or product.

A

Step 1 – Make sure the equation is balanced - This equation is already balanced.

Step 2 – Calculate the Mr of CaCO3 and the Mr of CO2.

Mr of CaCO3 = 100, Mr of CO2 = 44

Step 3 – You now know 100 g of CaCO3 would produce 44 g of CO2.

Step 4 – Find what 1 g of CaCO3 would produce.

(100/100) CaCO3 and 44/100 CO2

1g of CaCO3 would produce 0.44 g of CO2

Step 5 – Multiply both sides by 50 to work out what 50 g of CaCO3 would make.

50 g of CaCO3 would make (0.44 x 50g) of CO2

50 g of CaCO3 would make 22g of CO2

14
Q

Higher Q. Write a balanced symbol equation for the following reaction:

48 g of magnesium (Mg) reacts with 32 g oxygen (O2) to produce 80 g of magnesium oxide (MgO).

*this is just an example, you need to be able to do this for any equation when given the appropriate information.

A

Find ratio of moles in equation by finding number of moles of each substance using the equation:

Number of moles=(mass in g)/Mr

Mg: 48/24 = 2 ; O2: 32/32 =1 ; MgO: 80/40 = 2

Ratio = 2Mg: 1O2 : 2MgO

Balanced equation: 2Mg + O2 → 2MgO

15
Q

Higher Q. What is a ‘limiting reactant’?

A

The first reactant that gets used up in a reaction and causes the reaction to stop.

16
Q

Higher Q. Why is one reactant often added in excess to a reaction?

A

To ensure that all of the other reactant is used up.

17
Q

Higher Q. What are the two possible units for concentration?

A

g/dm3 or mol/dm3

18
Q

Higher Q. What equation links concentration, mass of solute and volume of solvent?

A

Concentration =(mass of solute)/(volume of solvent)

19
Q

Higher Q. What equation links concentration, number of moles of solute and volume of solvent?

A

Concentration =(number of moles of solute)/(volume of solvent)

20
Q

Higher Q. What is the mass of one mole of a substance, in grams, equal to?

A

The relative formula mass of the substance

21
Q

Higher Q. Calculate the concentration of sodium hydroxide solution when 44 g of sodium hydroxide is dissolved in 0.5 dm3 of water.

*this is just an example, you need to be able to do this for any equation when given a mass of a reactant or product.

A

44 g / 0.5 dm3

= 88 g / dm3

22
Q

Higher Q. Calculate the concentration of sodium hydroxide solution when 3.5 moles of sodium hydroxide is dissolved in 100 cm3 of water. Give your answer in mol/dm3.

*this is just an example, you need to be able to do this for any equation when given a mass of a reactant or product.

A

Convert volume of water to dm3

100 cm3 / 1000 = 0.1 dm3

3.5 moles / 0.1 dm3 = 35 mol/dm3

23
Q

Higher Q. What is the value of the Avogadro Constant?

A

6.02 x 1023 per mole