Molarity

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1. If some KHP is stuck in the scoopula when werighing the solud, the calculated wrong molarity of KHP will be lower than the expected value since the amount of water added has not changed (solvent) and the solute added was less hence the concentration will be lower therefore the molarity will be also lower.
2. If a little too much water is added, the concentration of the solution will be lower since the solvent in the solution will be more hence the calculated molarity will belower thatn the expected molarity value.
3. If there was an air bubble that came out when titrating KOH, its molarity will be higher than the actual molarity since the actual volume of KOH titrared will be lower than the volume calculated hnce the higher calculated molarity.
4. It is acceptable to add water as long as it is deionized
5. Adding water to any solution before titration is unacceptable because this will lead to dilution of the solution henc changing its molarity and concentration
6. If 25 ml of H₂SO₄ is titrated with NaOH, and requires 35.88 ml of 0.1127M NaOH, what is the molarity of the H₂SO₄   ?

H₂SO₄   + 2NaOH ®Na₂ SO₄ + 2H₂O

?                 0.1127M

25ml             35.88ml

For the equation to balance, the number of moles in the acid should be equal to the number of moles in the base

and, # of moles =

Hence, # of moles for NaOH is= =0.004044 moles

And M = =  = 0.1617 M

7. A sample of 0.495g solid KHP is weighed into an elernmeyer flask. This sample is titrated with a sodium hydroxide solution and 28.56 ml of NaOH are required to reach the endpoint. The sodium hydroxide solution is then used to titrate a sample of phosphoric acid of unknown concentration. It requires to use 29.88 ml of NaOH to reach with 10.33 ml H₃PO₄ What is the concentration of phosphoric acid?

 

KHC₈H₄O₄        +        NaOH®   NaKC₈H₄O₄ + H₂O

mass   0.495g

volume                                      28.56 ml

RFM    204

The no. of moles for KHP will be given by  no of moles =g/RFM

hence 0.495/204=0.0024264 moles of KHP

Since the no. of moles of KHP and NaOH are equal, the molarity of NaOH will be given by

= =  =0.08496

 

NaOH + H₃PO_4®