Conservation of mass

Question 1

law of conservation of mass

Answer 1

total mass of reactants and products stays constant during a chemical reaction

total mass before and after a reaction is the same

Question 2

closed system

Answer 2

situation in which no substances can enter or leave during a reaction

Question 3

examples of a closed system

Answer 3

• reactions in a sealed container, such as a flask with a bung
• precipitation reactions in a breaker

Question 4

precipitation reactions

Answer 4

two soluble reactions form an insoluble product, the precipitate

(e.g. NaCl(aq) + AgNO3(aq) -> NaNO3(aq) + AgCl(s))

total mass of the beaker and its contents stays the same during the formation of the silver chloride precipitate

Question 5

non-enclosed systems

Answer 5

situation in which substances can enter or leave during a reaction

Question 6

examples of a non-enclosed system

Answer 6

-reactions in an open flask, where a substance in the gas state may enter or leave

Question 7

non-enclosed system reaction of a reactive metal

Answer 7

mass is conserved

-mass of a reactive metal INCREASES if heated in air

(oxygen atoms combine with metal -> metal oxide)

Question 8

non-enclosed system reaction of a reactive non-metal/fuel

Answer 8

mass is conserved

-mass of a reactive non metal/fuel DECREASES if heated in air

(products in the gas state escape from the container)

Question 9

non-enclosed system reaction of a metal carbonate

Answer 9

mass is conserved

-mass of a metal carbonate DECREASES if it is heated

(CO2 gas is produced and escapes from the container)

Question 10

0.5g of lithium is added to water.

Calculate the mass of hydrogen gas produced.

(Ar: Li = 7, H = 1)

Answer 10

equation:
2Li + 2H2O -> 2LiOH + H2

(for the substances in the q, use the Ar to find the ratio)

2 x 7 -> 1 x (1+1)

14 2

14g of Li -> 2g of H2

(find the amount for 1g of Li)

1g -> 2/14 = 0.143g

(find the amount for 0.5g, in the question)

0. 5g -> 0.143g x 0.5 = 0.072g
1. 072g

(the Ar is used because the equation, like a molecular formula, tells you that for 2 lithium atoms 2 hydrogens are produced: 2:2
substituting the Ar in gets you the relative masses, 2 x 7 = 14 and 2 x 1 = 1)

Question 11

Methane, CH4, burns in oxygen to form carbon dioxide and water.

CH4 + 2O2 -> CO2 + 2H2O

Calculate the mass of oxygen used when 10g of methane burns.

(Ar: C = 12, H = 1, O = 16)

Answer 11

Mr of CH4: 12 + (4 x 1) = 16
Mr of O2 = (2 x 16) = 32

10g of CH4 reacts with (2 x 32/16) x 10g of O2
= 40g

Question 12

Potassium reacts with chlorine to form potassium chloride, KCl.

Calculate the mass of potassium neded to produce 20g of potassium chloride.

(Ar: K = 39, Cl = 35.5)

Answer 12

2K + Cl2 -> 2KCl

Ar of K = 39 and Mr of KCl = 39 + 35.5 = 74.5

20g of KCl is produced from (39/74.5) x 20g of K
=10.5g or 10.47g