From empirical formula to molecular formula ()

In summary, lactic acid has an empirical formula of HC3H5O3. It requires 17.0 mL of 0.65 M NaOH to reach the end point titration.
  • #1
francisco
18
0
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
 
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  • #2
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.
 
  • #3
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
 
  • #4
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?
 
  • #5
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.
 
Last edited:
  • #6
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 =).
 
  • #7
i see now. thanks!

francisco
 

What is the difference between empirical formula and molecular formula?

The empirical formula is the simplest ratio of elements in a compound, while the molecular formula provides the exact number of each element in a molecule. For example, the empirical formula for water is H2O, while the molecular formula is also H2O.

How do you calculate the empirical formula from experimental data?

The empirical formula can be calculated by dividing the mass or weight percentage of each element by its atomic weight and then converting the resulting ratios into whole numbers. These whole numbers represent the subscripts in the empirical formula.

What is the significance of the empirical formula in chemistry?

The empirical formula is important in determining the basic composition of a compound and can be used to predict the properties and behavior of the compound. It also helps in identifying and distinguishing between different compounds with similar molecular formulas.

Can the molecular formula be determined if only the empirical formula is known?

No, the molecular formula cannot be determined solely from the empirical formula. Additional information, such as the molar mass or mass spectrum, is needed to determine the exact number of atoms in the molecule and therefore the molecular formula.

What is the relationship between empirical formula and molecular formula for ionic compounds?

For ionic compounds, the empirical formula is the same as the molecular formula because the compound exists as a single molecule. This is because ionic compounds are made up of ions that are held together by ionic bonds, rather than individual atoms held together by covalent bonds.

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