What mass of sodium hydroxide would be present in 100 L of the waste water?

The concentration of sodium hydroxide in waste water from an alumina refinery was found by titrating 20.00 mL aliquots of waste water against 0.150 M hydrochloric acid, using phenolphthalein as indicator. The average titre of several titrations was 11.40 mL.

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First write a balanced chemical equation to see what is happening. this is a neutralization reaction between an acid (HCl) and a base (NaOH).

HCl + NaOH --> NaCl + HOH

The equation tells you that one mole of acid will combine with one mole of base to produce one mole of salt plus one mole of water.

To find the amount of HCl used you need to understand that molarity (M) is the number of moles of a substance in one liter of water.

M = moles/liter

If you rearrange this equation, you find that:

M * L = moles

In your problem, you have 0.15 M HCl and a volume of 11.4 mL or 0.0114 L.

So you have 0.15 * 0.0114 moles of HCl used

From the chemical equation, that means that in the 20 mL you had 0.0114 moles of NaOH.

If you had that amount in 20 mL, then you will have five times as much in 100 mL.

So calculate the total moles in 100 mL, convert that to mass and you have your answer.

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