The amount of pure HCl gas, in mol, that must be added to the solution to lower the pH from 12.0 to 2.0 would be
A - VCE - SSCE Chemistry - Question 16 - 2007 - Paper 1
Question 16
The amount of pure HCl gas, in mol, that must be added to the solution to lower the pH from 12.0 to 2.0 would be
A. 10
B. 2.0
C. 0.02
D. 0.01
Worked Solution & Example Answer:The amount of pure HCl gas, in mol, that must be added to the solution to lower the pH from 12.0 to 2.0 would be
A - VCE - SSCE Chemistry - Question 16 - 2007 - Paper 1
Step 1
Calculate the initial and final concentrations of H+ ions
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Answer
The initial pH is 12.0, which corresponds to a hydrogen ion concentration of
[H+]i=10−12extmol/L.
The final pH is 2.0, which corresponds to a hydrogen ion concentration of
[H+]f=10−2extmol/L.
Step 2
Determine the change in concentration
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Answer
The change in concentration of H+ ions needed is given by:
Δ[H+]=[H+]f−[H+]i=10−2−10−12≈10−2extmol/L.
Step 3
Calculate the amount of HCl required
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Answer
To completely neutralize the solution and achieve the final concentration of hydrogen ions, it is critical to factor in the volume of the solution. Assuming we have a volume of 1 L:
The amount of HCl gas needed can be calculated as:
Amount of HCl=Δ[H+]×Volume=10−2imes1=0.01extmol.
Step 4
Select the correct answer
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Answer
The amount of pure HCl gas that must be added to lower the pH from 12.0 to 2.0 is 0.01 mol, corresponding to option D.
