lactic acid is the avid responsible for the sour test in spit milk. on the basis of combustion analysis, it's percentage composition by mass is 40.00% carbon, 6.71%hydrogen and x% oxygen.
a. Calculate the empirical formula of lactic acid
b. The molar mass of lactic acid is 90g/mol. Determine the molecular formula
So I know the first step would be to find the # of g? And then concern that into moles then find the ratio between moles. And then for b you would find the g/mol in the empirical formula and divide it by the molar mass to find how many fit within the molecular formula.
So b makes sense to me, but I'm not sure how to go from the % to the number of g with that little information. Is it just 40g of C and 6.71g of hydrogen, then add those and take away a 100 to find O? That's all I can think of doing, but I don't get why.
Thank you very much
You have it.
Take a 100 g sample which gives you 40 g C, 6.71 g H and (100-40-6.71) g O.
You go from there. Post your work if you get stuck.
To find the empirical formula of lactic acid, you are correct that the first step is to determine the number of grams (g) of each element based on the given percentage compositions.
Let's assume we have a 100g sample of lactic acid. From the information given, we know that the percentage composition by mass is 40.00% carbon, 6.71% hydrogen, and x% oxygen.
a. Calculate the empirical formula of lactic acid:
To convert the given percentages to grams, you need to assume a specific mass for the sample. For simplicity, let's assume we have a 100g sample.
Carbon:
Percentage composition of carbon = 40.00%
Mass of carbon in the 100g sample = (40.00/100) * 100g = 40g
Hydrogen:
Percentage composition of hydrogen = 6.71%
Mass of hydrogen in the 100g sample = (6.71/100) * 100g = 6.71g
Oxygen:
Since we know that the mass of the sample is 100g, we can determine the mass of oxygen by subtracting the sum of the masses of carbon and hydrogen from the total mass:
Mass of oxygen in the 100g sample = 100g - (40g + 6.71g) = 53.29g
Now we have the masses of carbon, hydrogen, and oxygen in the 100g sample. To find the empirical formula, we need to convert these masses to moles and determine the ratio of the elements.
Carbon:
Molar mass of carbon = 12.01 g/mol
Number of moles of carbon = mass of carbon / molar mass of carbon = 40g / 12.01 g/mol ≈ 3.332 mol
Hydrogen:
Molar mass of hydrogen = 1.008 g/mol
Number of moles of hydrogen = mass of hydrogen / molar mass of hydrogen = 6.71g / 1.008 g/mol ≈ 6.647 mol
Oxygen:
Molar mass of oxygen = 16.00 g/mol
Number of moles of oxygen = mass of oxygen / molar mass of oxygen = 53.29g / 16.00 g/mol ≈ 3.331 mol
Next, we need to find the simplest whole-number ratio of the moles of each element. In this case, the ratio is approximately 1:2:1.
Therefore, the empirical formula of lactic acid is CH2O.
b. To determine the molecular formula of lactic acid:
Given the molar mass of lactic acid is 90 g/mol, we need to determine how many empirical formula units are present in one molecule of lactic acid.
Molar mass of empirical formula (CH2O):
= (12.01 g/mol × 1) + (1.008 g/mol × 2) + (16.00 g/mol × 1)
= 12.01 g/mol + 2.016 g/mol + 16.00 g/mol
= 30.026 g/mol
To find the number of empirical formula units, divide the molar mass of lactic acid by the molar mass of the empirical formula:
Number of empirical formula units = molar mass of lactic acid / molar mass of empirical formula
= 90 g/mol / 30.026 g/mol
≈ 2.997
Since we want a whole number ratio, we can round up to the nearest whole number, which is 3.
Therefore, the molecular formula of lactic acid is C3H6O.
Hope this helps!