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11.pH calculations weak acids

# 11.pH calculations weak acids - x 10-5 M pH =-Log[2.09 x...

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Acids Lesson 8 Weak Acids pH Calculations

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1. Calculate the pH of 0.45 M HCN. You need an ICE chart for weak acids or bases! HCN H + CN
The Ka is small , so x is small We will find that x = 0.000015 0.45 - 0.000015 = 0.45 This means we can make the approximation that 0.45 - x = 0.45 We can do this anytime the Ka has an exponent of 10 -4 or less Ka = x 2 = 4.9 x 10 -10 0.45 - x

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x 2 = 4.9 x 10 -10 0.45 x = [H + ] = 0.000014849 M pH = -Log[0.000014849] pH = 4.83 2 sig figs due to molarity and Ka
2. Calculate the pH of 0.60 M H 3 BO 3 (Triprotic acid) (To calculate pH only first step is significant) H BO H + H BO 0 small ka x 2 = 7.3 x 10 -10 0.60

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x 2 = 7.3 x 10 -10 0.60 x = [H + ] = 2.09

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Unformatted text preview: x 10-5 M pH =-Log[2.09 x 10-5 ] pH = 4. 68 2 sig figs due to molarity and Ka 3. Calculate the pH of a 0.20 M diprotic acid with a Ka 1 = 4.7 x 10-7 H X ⇄ H + HX 0 small ka x 2 = 4.7 x 10-7 0.20 x = [H + ] = 3.066 x 10-4 M pH =-Log[3.066 x 10-4 ] pH = 3. 51 2 sig figs due to molarity and Ka 4. Calculate the pH of a saturated solution of Mg(OH) 2 . Magnesium hydroxide is slightly soluble in water so its pH depends on its Ksp value This is a solubility equilibrium- no ICE Mg(OH) ⇄ Mg + 2OH 2x = [OH-] = 2.237 x 10-4 M pOH = 3.65 pH = 10.35...
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11.pH calculations weak acids - x 10-5 M pH =-Log[2.09 x...

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