Question

Tear this page out of your laboratory manual and submit it to your TA with your answers. B. Post-Lab Assessment Questions: (20 pts) 1. A reaction between sodium hydroxide and hydrochloric acid produces sodium chloride and water. a) (2 pts) Write the balanced chemical equation for this reaction. b) (3 pts) If 22.85 g of sodium hydroxide reacts completely with 20.82 g of hydrochloric acid to form 10.29 g of water, what mass of sodium chloride is formed during the reaction? 2. (8 pts) When a solution of barium nitrate and a solution of copper (II) sulfate are mixed, a chemical reaction produces solid barium sulfate and an aqueous solution of copper (II) nitrate. An unknown amount of barium nitrate is dissolved in 120.0 g of water and 8.15 g of copper (II) sulfate is dissolved in 75.0 g of water. When these solutions are mixed 1.076 g of white solid is isolated as a precipitate by vacuum filtration. The filtrate is found to have a mass of 204.44 g. What mass of barium nitrate was dissolved in the water and used for this reaction? You must show both the balanced chemical equation and your calculations to receive credit for this problem. 3. (3 pts) When a wooden log burns it is essentially undergoing a combustion reaction. Using this information and what you know about mass conservation, explain why the ashes that form when a log burns have a lower mass than the log had before it started burning 4. (4 pts) Consider the following chemical reaction: CuSO4 (s) + heat → CuO (s) + S03 (g) Design an experiment in which you use this chemical reaction to test the law of mass conservation.

Question 4.

Answer 1

1. a) NaOH + HCl = NaCl + H₂O

b) Considering conservation of mass, the amount of sodium chloride produced = [(22.85 +20.82) - 10.29]

= 33.38 g

2. The substitution reaction, Ba(NO₃)₂ (aq) + CuSO₄ (aq) = BaSO₄ (s) + Cu(NO₃)₂(aq)

Before reaction , Mass of barium nitrate solution = (x+ 120) g; Mass of copper sulphate solution = (8.15+75)g

After reaction, mass of residue = 1.076 g and mass of filtrate = 204.44 g

Therefore, x = (204.44+1.076) g -(120+83.15)g = 2.336 g (applying conservation of mass)

So, amount of barium nitrate added = 2.336 g.

3. Combustion of wooden log produces carbon dioxide and water vapour with the release of high heat energy and light energy.

Wooden log + Dioxygen = Carbon dioxide + Water vapour + Heat energy + Light Energy.

Before the combustion reaction if we measure exact quantity of wooden log and dioxygen taken and if we able to measure the masses of produced carbon dioxide and water vapour and the mass equivalent of produced heat and light energy, then it will follow the conservation of mass.

The ash contains only rest part of component present in the wooden log except carbon and hydrogen, as it contains lignin (cellulose) a polysaccharide with some nitrogen, sulphur etc. on complete combustion. So, it weighs little compared to the initially taken wooden log.

4. In a sealed tube measured quantity of copper sulphate is taken. On heating it undergoes thermal decomposition to produce sulphur tioxide and copper oxide. Finally if we measure the mass of the sealed tube we obtain the same value of mass as before, indicating conservation of mass.