pH, pKa, Ka, pKb, Kb
Key Questions
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#"p"K_"a"# and#"p"K_"b"# are measures of the strengths of acids and bases, respectivelyAcids
When you dissolve an acid in water, it undergoes an equilibrium reaction with the water in an.
HA + H₂O ⇌ H₃O⁺ + A⁻
The value of the equilibrium constant is given by
#K_"a" = (["H"_3"O"^+]["A"^-]]/["HA"]# The greater the value of
#K_"a"# , the stronger the acid.For most weak acids,
#K_"a"# ranges from#10^-2# to#10^-14# .We convert these exponential numbers into a normal range by taking their negative logarithm.
The operator
#"p"# means "take the negative logarithm of".So
#"p"K_"a" = -logK_"a"# .For most weak acids,
#"p"K_"a"# ranges from 2 to 14.Thus, the smaller the value of
#"p"K_"a"# , the stronger the acid.Bases
When you dissolve a base in water, it reacts with the water in an equilibrium reaction.
B + H₂O ⇌ BH⁺ + OH⁻
The value of the equilibrium constant is given by
#K_"b" = (["BH"^+]["OH"^-]]/["B"]# The greater the value of
#K_"b"# , the stronger the base.For most weak acids,
#K_"b"# ranges from#10^-2# to#10^-13# .#"p"K_"b" = -logK_"b"# .For most weak acids,
#"p"K_"a"# ranges from 2 to 13.The smaller the value of
#"p"K_"b"# , the stronger the base.Here's a video on
#"p"K_"a"# and#"p"K_"b"# .
Ernest Z.
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6
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Dec 20 2014
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Answer:
These are measures of acidity and basicity...
Explanation:
And acid in aqueous solution is conceived to undergo a protonolysis reaction...
#HX(aq) + H_2O(l) rightleftharpoonsH_3O^+ + X^(-)# And as for any equilibrium, we can measure and quantify it in the usual way...
#K_a=([H_3O^+][X^-])/([HX(aq)])# Note that
#H_2O# DOES NOT appear in the equilibrium expression because it is present in such high concentration that it is effectively constant..For strong acids, i.e.
#HI# ,#HBr# ,#HCl# ,#H_2SO_4# protonolysis is effectively quantitative: the given equilibrium lies entirely to the right as we face the page, and the acid solution is quantitative in#H_3O^+# . For weaker acids,#HF# ,#H_3C-CO_2H# , the equilibrium lies somewhat to the left...and concentrations of the parent acid remain at equilibrium.And likewise, we can formalize the performance of a base by an equivalent equilibrium...we use ammonia, because this is a WEAK base in aqueous solution...
#NH_3(aq) + H_2O(l) rightleftharpoonsNH_4^+ + HO^-# And
#K_b# is defined in an equivalent way to#K_a# ...#K_b=([NH_4^+][HO^-])/([NH_3(aq)])# ,#K_b"(ammonia)"=1.74xx10^-5# ...Confused yet....?
Well, note that NECESSARILY....for a given acid/conjugate pair, say
#NH_4^+"/"NH_3# ...#K_aK_b=10^-14# ...or perhaps more usefully...#pK_a+pK_b=14# ...
anor277
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1
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Jun 18 2018
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The pH scale provides a way of measuring how acidic or basic solutions are. The scale ranges from 0-14. A pH of 0 is the most acidic, 7 is neutral and 14 is the most basic.
Here is a video of a lab which looks at a number of different solutions and measures their pH levels using a pH meter and an indicator.
video from: Noel Pauller
mrpauller.weebly.com
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Jan 9 2015