Post-Lab Questions: 1. Compare and explain the observed values of pH for these t
ID: 526231 • Letter: P
Question
Post-Lab Questions:
1. Compare and explain the observed values of pH for these two acids.
2. Which acid pH changed the most on dilution? Explain why (consider the equilibrium occurring).
Strong Acid [HCl]1 [HCl]2 Measured pH 1.36 2.87 Molarity 0.10 M 0.010 M Calculated pH 1.00 2.00 A. pH of Acid Solutions: 1. Strong Acid, HCl: Obtain 10.0 mL of 0.10 M HCl. Transfer 9.0 mL to a 50 mL or 100 mL beaker labeled [HCl,. Dilute 1.0 mLof the 0.10 M HCl to 10.0 mL with deionized water in your graduated cylinder, transfer to a 50 mL or 100 mL beaker, and label [HCl]2. Measure the pH of each solution. 2. Weak Acid, HCH,0; Obtain 10.0 mL of 0.10 M HC H,C, Transfer 9.0 mL to a 50 mL or 100 mL beaker labeled [HCH,02] Dilute 1.0 mL of the 0.10 M HC H,0, to 10.0 mL with deionized water in your graduate cylinder, transfer to a 50 mL or 100 mL beaker, and label it [HC2H,02)2. Measure the pH of each solution.Explanation / Answer
1. For HCl the value of the pH doubled when the concentration was reduced ten times. This is because HCl is a strong acid and follows the formula:
pH = -log(H+)
From this formula, we get from the log (which is base 10) that in fact, the pH will be doubled when the concentration is reduced ten times.
Note: in strong acids, H+ represents the acid concentration because the acid completely ionizes in water.
Now, acetic acid doesn't follow this tendency because it is a weak acid and it follows the same formula, however, not all the acid ionizes in water and we must calculate (H+) first with the formula:
(H+) = (Ka*C)1/2
pH = -log(H+)
So, since not all the acid ionizes, the pH doesn't change that abruptly as if it was a strong acid.
2. Since weak acids don't ionize completely, the pH doesn't change abruptly when it is diluted, in fact, some weak acids are used as buffer (in a mix with salts) because they can stabilize the pH of a system and keep it constant. HCl changed its pH the most on dilution because it is a strong acid.