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Amphoteric Solutions A chemical compound able to react with both an acid or a base is amphoteric. Water is amphoteric. The two acid-base couples of water are H 3 O + /H 2 O and H 2 O/OH - It behaves sometimes like an acid, for example And sometimes like a base : Hydrogen carbonate ion HCO 3 - is also amphoteric, it belongs to the two acid-base couples H 2 CO 3 /HCO 3 - and HCO 3 - /CO 3 2-

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Neutralization Problems If an acid and a base combine in a 1 to 1 ratio, then the volume of the acid multiplied by the concentration of the acid is equal to the volume of the base multiplied by the concentration of the base

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V a M a = V b M b V b = volume of the base (in L) M b = concentration of the base (in moles/L) V b M b = L x moles/L = moles of base V a = volume of the acid (in L) M a = concentration of the acid (in moles/L) V a M a = L x moles/L = moles of acid V b M b = V a M a is the same as Moles of base = Moles of acid!

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Example: You have 25 mL of HCl, with an unknown concentration. It takes _____ mL of 1.0 M NaOH to titrate (or neutralize) the acid. What is the concentration of the HCl? Tip: figure out what the variables are here (V b, M b, V a, and M a ). Figure out what youre solving for, and then solve!

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Answer: You have 25 mL of HCl, with an unknown concentration. It takes _____ mL of 2.0 M NaOH to titrate (or neutralize) the acid. What is the concentration of the HCl?

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What is titration? Quantitative method used to find the concentration of a substance.

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Finding Unknown Solution Standard Solution – 0.1M HCl Titrant – unknown concentration NaOH Indicator – Phenolphthalein End point – Where the indicator changes color We are trying to reach the equivalence point Where moles Acid = moles Base In our lab, equivalence point and end point are the same!

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What does this mean? If you use 100mL of NaOH that has a concentration of 0.1 moles/L, how many moles of base have you used? L x 0.1 moles/L =.01 moles of base So how many moles of acid were in the unknown acid?.01 moles!! Because moles base = moles acid in a titration.

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Neutralization Problems Example 1: Hydrochloric acid reacts with potassium hydroxide according to the following reaction: HCl + KOH KCl + H 2 O If mL of M HCl exactly neutralizes mL of KOH solution, what is the concentration of the KOH solution? Solution: V acid C acid = V base C base (15.00 mL )(0.500 M) = (24.00 mL ) C base C base = (15.00 mL )(0.500 M) (24.00 mL ) C base = M

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Neutralization Problems Whenever an acid and a base do not combine in a 1 to 1 ratio, a mole factor must be added to the neutralization equation n V acid C acid = V base C base The mole factor (n) is the number of times the moles the acid side of the above equation must be multiplied so as to equal the base side. (or vice versa) Example H 2 SO NaOH Na 2 SO H 2 O The mole factor is 2 and goes on the acid side of the equation. The number of moles of H 2 SO 4 is one half that of NaOH. Therefore the moles of H 2 SO 4 are multiplied by 2 to equal the moles of NaOH.

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Titrations V a M a = V b M b What the HECK is that?!?!?!?

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