Question

Calculate how many lactic acid monomers are in a 5g sample of PLA (Polylatic acid). (Hint: what is the ‘molar mass’ of the repeated monomeric unit in PLA?) Use the answer to calculate how much NaOH you will need for the saponification and how much HCl you will need to product lactic acid from sodium lactate. ​

Answer 1

note that

MW of lactic acid = 90.08 g/mol

so..

moles in 5 g--> mass/MW = 5/90.08 = 0.05550621 moles of Lactic acid...

1 mol = 6.022*10^23

so..

0.05550621 mol of acid = 0.05550621*6.022*10^23 = 3.342*10^22 monomers of LActic acid in polylactic acid

1 mol of NAOH --> 1 monomer

so...

mol of NAOh = 0.05550621 mol

mass of NAOH = mol*MW = 0.05550621*40 = 2.220 g of NaOH required

for HCl:

ratio is also 1:1

so

mass of HCL = mol*MW = 0.05550621*36= 1.9982 g of NaOH required