Acids, Bases and Salts Questions and Answers (Page 1)
Dry hydrogen chloride gas does not turn blue litmus paper red. It dissolves in water and toluene. Some properties of the two solutions are listed below:
|HCl in toluene||HCl in water|
|Nuetral to litmus||Turns litmus paper red|
|Does not conduct electricity||Conducts electricity|
|Does not react with zinc||Gives off hydrogen with zinc|
a) Which solution is acidic?
b) Why does one solution conduct electricity while the other does not?
c) What are the products likely to be at the anode and at the cathode when a solution of hydrogen chloride in water is electrolysed?
d) How would the solution of HCl in water react with sodium carbonate solution?
e) Some water, pH = 7, is added to a few cm3 of the solution of hydrogen chloride in toluene and the mixture shaken. After a few seconds, two layers can be seen in the tube. Universal indicator paper is added to the lower (water) layer. What would you observe?
a) HCl in water.
because it turns litmus paper red.
b) A solution of HCl gas in water conducts electricity because water is a polar solvent, so when HCl gas dissolves in water it forms ions which carries charge throughout the liquid.
A solution of HCl gas in toluene does not conduct electricity because toluene is a non polar solvent, so when HCl gas dissolves in toluene it does not form ions.
c) Hydrogen gas at the cathode. Oxygen gas at the anode.
d) Sodium carbonate would dissolve a solution of HCl in water releasing bubbles of carbon dioxide.
e) The universal indicator will turn red.
Lemon juice may be considered to be a dilute aqueous solution of citric acid. If you had a bottle of lemon juice, how would you show that citric acid is a weak acid?
Test it with universal indicator and the universal indicator will turn yellow.
The preparation of zinc sulphate can be represented by the equation:
ZnCO3(s) + H2SO4(aq) → ZnSO4(aq) + H2O(l) + CO2(g)
a) How would you mix the reagents?
b) How would you know when there is no more unreacted acid?
c) How would you remove the excess solid?
d) How would you obtain crystals of zinc sulphate?
e) 1.25g of zinc carbonate are dissolved in excess acid. What mass of zinc sulphate is obtained from the resulting solution?
a) Slowly add the powdered zinc to a beaker of dilute sulphuric acid.
b) Continue adding the zinc powder until no more zinc powder dissolves.
c) Filter off the excess zinc.
d) Heat the filtrate until you obtain a saturated solution, then leave the saturated solution in an evaporation basin until crystals of zinc sulphate form.
e) According to the reaction, 1 mole of zinc carbonate reacts to produce 1 mole of zinc sulphate.
Now lets how many moles is 1.25g of zinc carbonate.
N = Mass/Mr
= 1.25/(65 + 12 + 48)
= 0.01 moles
Since the molar ratio of zinc carbonate: zinc sulphate = 1:1
This also means that 0.01 moles of zinc carbonate produces 0.01 moles of zinc sulphate.
Number of moles (H2SO4) = Mass/Mr
0.01 = M/(65 + 32 + 64)
0.01 = M/161
M = 1.61 g
Mass of zinc sulphate produced = 1.61g