21 Acid-Base Equilibrium Problems

21 Acid-Base Equilibrium Problems - Ammonium nitrate is...

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1 1 Ammonium nitrate is used in fertilizers. Calculate the pH of a 0.15 M solution of NH 4 NO 3 . Recognize that ammonium nitrate is a salt of the polyatomic ions NH 4 + and NO 3 - . Recognize that ammonium is a weak acid, and look up that its K a = 5.6x10 -10 . Set up the RICE table and use the K a expression. A) 5.04 B) 4.05 C) 6.50 D) 2.09 E) 5.60 2 Solution pH = -log[H 3 O + ] = 5.04 ++ 42 33 -7 NH +H O NH +H O 0.15 M 0 1x10 M -x +x +x 0.15-x x x   2 10 a 2- 1 0 -6 3 x K = 5.6x10 .15-x x =(0.15)(5.6x10 ) x=9.2x10 [ ] HO Check that the assumption is OK. x < 0.15 M/20. If not, need to solve the quadratic equation using the magic formula. Since Ka is smaller than 10 -3 expect x to be small is assume it is negligible compared to 0.15 M.
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2 3 Relative strengths of acids and their conjugate bases. Last time we found for all conjugate acid-base pairs K a x K b = K w pK a + pK b = pK w = 14 at 298 K There is an inverse relationship between K a and K b and between pK a and pK b In comparing two weak acids, e.g. K a =10 -3 & K a =10 -5 The stronger acid has the larger K a and the smaller pK a . Acids with the larger K a will have conjugate bases with the smaller K b i.e. the stronger weak acids have the weaker conjugate base You need to remember all this or have it written down, and understand how to think about these comparisons. 4 Why does it make sense that the stronger acid has the weaker conjugate base?
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21 Acid-Base Equilibrium Problems - Ammonium nitrate is...

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