I can not figure out where I am calculating wrong so I need detailed work shown

Question

I can not figure out where I am calculating wrong so I need detailed work shown please.

In the titration of an unknown amino Acid, I have to calculate the molar mass (grams amino acid in 1 mole of amino acid) of the unknown by two methods:

Know data:

Unknown Amino acid is 15.00g/L

We used a 20.mL sample of Amino Acid

Titrated with 0.499 M NaOH

Method One: Vol of NaOH required to get from initial pH to 2nd Equivalence Point:__(12.46mL) or 0.01246 L

molar mass = _______g/mol (ratio here is 2 mol NaOH to 1 mole Amino acid)

Method Two: Vol of NaOH required to get from the 1st to the 2nd Equivalence point:___(6.96mL) or 0.00696L

molar mass = _______g/mol ( ratio here is 1/1)

It also asks which method is more reliable, and to give two reasons why.

Thanks

Explanation / Answer

1.Molarity of NaOH = 0.499M

Volume used = 12.46ml

0.499M means 0.499 mol in 1000 ml

So,in 12.46ml, Mol of NaOH = (0.499/1000)×12.46ml=0.00622mol

2mol NaOH react with 1mol of amino acid

Therefor mol of amino acid reacted is 0.00622/2=0.00311mol

0.00311mol present in 20ml ( volume taken for titration )

Therefore, mol present in 1000ml ( 1litre) is (0.00311/20)×1000 =0.1555mol

in 1litre 15 g of amino acid present and this 15g equal to 0.1555 mol of amino acid

Mol = mass /molar mass

Molar mass = Mass /mol

Therefore, molar mass of amino acid = 15g/0.1555 = 96.46g/mol

2. Volume of NaOH consmed =6.96ml

By calculating similar to above

mol of NaOH = (0.499mol/1000ml)×6.96ml =0.00347mol

1mol NaOH react with 1mol of 1mol of Amino acid

Therefore, no of mol of amino acid reacted is 0.00347mol which is present in 20ml

Thereore , Mol present in 1000ml of amino acid solution = (0.00347/20)×1000 = 0.1737mol

Therefore, molar mass = 15g/0.1737mol=86.355g/mol

3.Method 1 is more reliable because

a) Equivalence point is differ from end point of the the titration , equivalence point is just the stoichiometry amount added point but because of equillibrium the acidity not completely neutralized.So, if we take first equivalence point as reference it will give less acurate value.

b) Volume consumed in second method is less,generally ,in titration method,for better acurate result volume consumed should be 10ml to 25 ml.

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