From empirical formula to molecular formula ()

[francisco]

i also need help with the following problem:

lactic acid (found in sour milk) has an empirical formula HC3H5O3.

a 1.0-g sample of lactic acid requires 17.0 mL of 0.65 M NaOH (sodium hydroxide) to reach the end point titration.

what is the molecular formula of lactic acid?

thanks.

francisco

 

[ksinclair13]

To find the molecular formula, you must first find the molecular mass from the data given. Then, take the molecular mass and divide it by the mass of HC3H5O3. Approximate that to the nearest whole number and then multiply the empirical formula by that number to get your molecular formula.

Try to figure it out and post your work so that we can help you further.

 

[francisco]

molecular weight = 1.00794 g H + 3*(12.0107 g C) + 5*(1.00794 g H) + 3*(15.9994 g O) = 90.8 g HC3H5O3.

17.0 mL*(L/1000 mL)*(0.65 mol NaOH/L) = 0.011 mol NaOH.

since to reach the end point titration means to reach the point at which the number of moles of NaOH is equal to the number of moles of HC3H5O3, the number of moles of HC3H5O3 is 0.011.
0.011 mol*(90.8 g HC3H5O3/mol) = 0.991 g HC3H5O3 (the mass of HC3H5O3).

(90.08/0.991) = 90.9 (rounding to nearest whole is 91).

how do i multiply 91 by HC3H5O3 to get the molecular formula?

thanks

francisco

 

[ksinclair13]

Not quite...

Think about it this way. It took .011 moles of NaOH to reach the end point ( .017 L * (.65 mol/L) ). At the end point, there are equal amounts of moles of the acid and base. Therefore, 1.0 g * (1 mol / X grams) = .011 moles. Now, solve for X and you have your molecular mass. Then continue as before.

Do you follow this?

 

[francisco]

1.0 g * (1 mol / X g) = 0.011 mol.

yes, i think i follow.

1 mol * (1.0 g / 0.011 mol) = 90.9 g.

is it wrong to use the empirical formula to find the molecular mass?
i would use the molecular formula to find the molecular mass, but the molecular formula is what is in question.

what then is the mass of HC3H5O3? is it 90.08?

i previewsly said the molecular weight (i meant molecular mass) = 1.00794 g H + 3*(12.0107 g C) + 5*(1.00794 g H) + 3*(15.9994 g O) = 90.8 g HC3H5O3. the number is wrong. it is not 90.8 g. it is 90.08 g. also, it is not the molecular mass. is it the mass of HC3H5O3?


(90.9 / 90.08) = 1.009.

i still do not understand.

thanks.

francisco.

 

[ksinclair13]

Yes, you're on the right track now. Now, round 1.009 to the nearest whole number and then multiply your empirical formula by that to get the molecular formula. In this case, they are the same =).

 

[francisco]

i see now. thanks!

francisco