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Solution: Strong acids dissociate in aqueous solutions completely and the
concentration of Hydrogen ions is equal to the concentration of acid:
+
–
НСl ↔ Н + Cl
+
[НСl] = 0,03 М and [Н ] = 0,03 М.
–2
+
So: рН = – lg [Н ] = – lg 3·10 = 2 – lg 3 = 1,5.
Answer: For 0.03 M solution НСl рН = 1,5.
Example 6. Determine the pH of a 0.01 M solution of NaOH. Solution:
NaOH is a strong base.
+
NaOH ↔ Na + OH –
–
[NaOH] = 0,01 М, and [OН ] = 0,01 М.
Consequently, pOH = - lg [OH-] = - lg 10-2 = 2
Taking into account that pH + pOH = 14, we get
рН = 14 – рОН = 14 – 2 = 12.
Answer: For 0,01 M of NaOH solution pH equals 12.
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