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The taste of sour milk is lactic acid. What is the molecular formula for lactic acid if the percent composition is 40.00% C, 6.71% H, 53.29% O, and the approximate molecular mass is 90 amu?

User Scott Hunter
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2 Answers

28 votes
28 votes

Final answer:

The molecular formula for lactic acid is C3H6O3, which gives sour milk its taste. To determine the empirical formula, we use the given percent composition to calculate the number of moles of carbon, hydrogen, and oxygen. Dividing the mole values by the smallest mole value gives us the subscripts for the empirical formula. The molar mass of the empirical formula is then used to determine the molecular formula.

Step-by-step explanation:

The molecular formula for lactic acid, which gives sour milk its taste, is C3H6O3. To determine the molecular formula, we first need to calculate the empirical formula. Based on the given percent composition, we assume we have 100 grams of lactic acid. This gives us 40.00 grams of carbon, 6.71 grams of hydrogen, and 53.29 grams of oxygen.

Next, we convert the masses to moles using the molar masses of the elements. The molar masses are approximately:

C: 12.01 g/mol

H: 1.01 g/mol

O: 16.00 g/mol

Converting the masses to moles gives us:

Carbon: 40.00 g / 12.01 g/mol ≈ 3.33 moles

Hydrogen: 6.71 g / 1.01 g/mol ≈ 6.64 moles

Oxygen: 53.29 g / 16.00 g/mol ≈ 3.33 moles

Dividing each mole value by the smallest mole value (3.33 moles) gives us the subscripts for the empirical formula:

C1H2O1.

Finally, to find the molecular formula, we need to determine the molar mass of the empirical formula. The empirical formula has a molar mass of approximately 30.03 g/mol. Since the approximate molecular mass given is 90 amu, we divide it by the molar mass of the empirical formula:

90 amu / 30.03 g/mol ≈ 3

This tells us that the molecular formula is three times the empirical formula, giving us the molecular formula of C3H6O3 for lactic acid.

User Fravadona
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25 votes
25 votes

Answer:


C_3H_6O_3

Explanations:

Given the following parameters

Carbon = 40.0%

Hydrogen = 6.71%

Oxygen = 53.29%

Convert to mass

Carbon = 40.0grams

Hydrogen = 6.71grams

Oxygen = 53.29grams

Convert the mass to moles

Mole of carbon = 40.0/12 = 3.33moles

Mole of hydrogen = 6.71/1 = 6.71 moles

Mole of oxygen = 53.29/16 = 3.33moles

Divide by the lowest number of moles

Carbon: 3.33/3.33 = 1

Hydrogen: 6.71/3.33 = 2.01

Oxygen: 3.33/3.33 = 1

Determine the empirical formula

Empirical formula = CH2O

Determine the molecular formula


\begin{gathered} (CH_2O)_n=90 \\ (12+2(1)+16)n=90 \\ 30n=90 \\ n=(90)/(30) \\ n=3 \end{gathered}
(CH_2O)_3=C_3H_6O_3

Hence the molecular mass of the compound is C3H6O3

User Squeazer
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