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stoichiometric equation

Anyone have any idea how to answer this question?
In the practical, Lactobacillus plantarum was grown in a medium containing 5 g/L of lactose. Being a homoforementative lactic acid bacterium lactose is expected to be converted to lactic acid. What is the maximum lactic acid concentration possible? You need to use the stoichiometric equation to calculate this.

Is there any extra information needed for this, and is there anyway someone can talk me through it step by step I have been stuck for ages.
I'd really appreciate it!!
You need the equation which shows how many moles of lactose are converted to how many moles of lactic acid - i.e. what is the stoichiometric ratio?
The process that converts lactose (milk sugar) to lactic acid is fermentation, and is the reason for milk going sour (you might know that acids taste sour e.g. citric acid in an orange OR ascorbic acid (Vitamin C tablet).

This reaction is actually not so, it is a series of reactions involving breakdown of lactose ( a disaccharide - two hexoses joined together) into glucose and galactose (the 2 monosaccarides lactose is made from), then other complex reactions involving e.g. glucose-6-phosphate.

Relax though, you only need to know the overall reaction to answer this Q.

C12H22O11 + H2O ---------> 4C3H6O3


If conc-n of lactose (original given) is 5g/L:-

1 Take C12H22O11 and work out its molecular mass: (12 X 12) + (22 X 1) + (11 X 16) = 144 + 22 + 176 = 342 i.e,. one mole of lactose weighs 342g.

2. So, taking 1 litre of solution, (which will contain 5g of lactose), the number of moles of lactose in one litre = 5/342


3. Work out molecular mass of lactic acid:- C3H6O3: = (3 X 12) + (1 X 6) + (3 X 16) = 90.

BUT you see from the equation that one mole of lactose gves 4 moles of lactic acid.

So number of moles of lactic acid produced from one mole of lactose is four times no of moles of lactose used = (5/342) X 4.= 0.585 moles (in one litre)

The molecular mass of a substance means one mole weighs that much . So one mole of lactic acid weighs 90 g.

But we have 0.585 moles, so mass of lactic acid (AND DON'T FORGET, this is in one litre) = 0.585 X 90 = 52.65 g

So, if there is 52.65 g in one litre, the concentration of lactic acid produced = 52.65 g/l (ANSWER)

TRY AND UNDERSTAND THIS TACTIC for this type of Q:
1. You need a balanced equation.
2. Work out molecular masses.
3. Think to work out whatever you need to (no of moles OR mass in grammes, etc.)

If confused.com :smile: don't hesitate to post again.

Good luck

M (science tutor)

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