20) Balanced chemical equation: 2KNO₃ → 2KNO₂ + O₂.
Potassium nitrate is decomposed on potassium nitrite and oxygen.
According to principle of mass conservation, number of atoms must be equal on both side of chemical reaction.
There are two potassium atoms, two nitrogen atoms and six oxygen atoms on both side of balanced chemical reaction.
Decomposition is reaction where one substance (in this example potassium nitrate) is broken down into two or more simpler substances (potassium nitrite and oxygen).
21) The answer is:
Balanced chemical reaction: double recplacement reaction.
MgCl₂(aq) + Na₂CO₃(aq) → MgCO₃(s) + 2NaCl(aq).
The magnesium (Mg) and the sodium (Na) replace each other making this a double replacement reaction.
Reactivity series is an empirical progression of a series of metals, arranged by their reactivity from highest to lowest (alkaline metals have highest reactivity and Noble metals lowest reactivity):
K > Ba > Sr > Na > Ca > Mg > Be > Al > Mn > Zn > Cr> Fe > Cd > Co > Ni > Sn > Pb > H(in acids) > Cu > Hg > Ag.
Metal higher in the reactivity series will displace another and it will be oxidized.
Magnesium is higher it this series than aluminium and aluminium is higher than copper.
22) The answer is: half life is 22 years.
Half-life is the time required for a quantity (in this example mass of the element) to reduce to half its initial value and is independent of initial concentration.
Mass of the element is reduced by half in 22 years from 45 grams to 22.5 grams.
First calculate the radioactive decay rate constant λ:
λ = 0.693 ÷ t.
λ = 0.693 ÷ 22 y.
λ = 0.0315 1/y.
ln(m/m₀) = -λ·t₁.
ln(4.8125/308) = -0.0315 1/y · t₁.
t₁ = 132.02 y.
