A student pipets 2.000mL of cloudy ammonia solution into an empty 250.0mL Erlenm
ID: 894008 • Letter: A
Question
A student pipets 2.000mL of cloudy ammonia solution into an empty 250.0mL Erlenmyer flask. 25.00mL of 0.2000M HCl solution is immediately added. The solution is swirled. The resulting mixture is titrated with a solution of 0.05000 M Ca(OH)2. An equivalence point is found at 6.83mL of Ca(OH)2 solution. Determine the concentration of the original ammonia solution.
A. Determine the moles of calcium hydroxide added to reach the equivalence point in the titration.
B. Determine the moles of hydrochloric acid were consumed during the titration.
C. Determine the original moles of hydrochloric acid that were added to the ammonia solution.
D. Determine the moles of hydrochloric acid that reacted with the ammonia.
E. Determine the original moles of the ammonia.
F. Determine the original molarity of the ammonia solution.
Explanation / Answer
reactions:
HCl(aq) + NH3(aq) NH4Cl(aq)
2 HCl + Ca(OH)2 ---> 2 H2O + CaCl2
original moles of HCl added to the ammonia = 0.2 M * 0.025 L = 0.005 moles
HCl(aq) + NH3(aq) NH4Cl(aq)
0.005 x
Moles of CaOH2 added to reach the equivalence point:
0.05 M * 0.00683 L = 0.00034 moles
There are 2 moles of HCl per mole of CaOH2, so:
moles of HCl consumed during the titration = 2*0.00034 = 0.00068 moles
moles of HCl that reacted with the ammonia: 0.005-0.00068 = 0.00432
There is 1 mol of ammonia per mol of HCl, so:
original moles of ammonia: 0.00432
original molarity of the ammonia solution= 0.00432 moles / 0.002 L = 2 M
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