Question

A chemical equation is shown below. KNO3 → KNO2 + O2 What are the coefficients that should be added to balance this equation? Use complete sentences to explain your answer. Explain how this chemical reaction demonstrates the conservation of mass. Question 21 (Essay Worth 8 points) (04.03 MC) Part 1: Name the type of chemical reaction that occurs when magnesium chloride solution (MgCl2) reacts with sodium carbonate solution (Na2CO3). Part 2: Explain why aluminum (Al) would react with copper chloride (CuCl2) but not with magnesium chloride (MgCl2). Question 22 (Essay Worth 8 points) (04.07 HC) The table shows the amount of radioactive element remaining in a sample over a period of time. Radioactive Decay Rate Amount of Radioactive Sample (grams) Time (years) 45.0 0 31.8 11 22.5 22 15.9 33 11.3 44 Part 1: What is the half-life of the element? Explain how you determined this. Part 2: How long would it take 308 g of the sample to decay to 4.8125 grams? Show your work or explain your answer.

Answer 1

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.