How do you find the molecular and empirical formulas?

Answer 1

There is no easy answer to this question. First, it depends what information you have to start with. Do you have the name? Do you know what elements are found in the substance?

If you are given the name of the molecule, you can often find the molecular structure of that compound by simply using the Google search engine online and searching for the name. It is often give you the answer! There are certain compounds that you should be familiar with also. Also, there is a set of official rules that determine the name of a specific compound, so that if you have the formula, you can name it correctly, and if you have the name, you can determine the formula. Unfortunately, many chemicals also have common names that do not follow these rules, and you just have to memorize those, or look them up with Google.

See the Related Questions and Web Links to the left for some information about how compounds are named and how to go from a formula to a name and vice versa.

Answer 2

You need additional information to go from the molar mass to the molecular formula. You need to know something about what elements are present in the compound and often you need to know what the percent composition of the compound is of each element. This is done by something called elemental analysis.

For example, let's say you have a compound with molecular weight of 44 grams per mole. That could be C3H8 (propane) or CO2 (carbon dioxide). Without knowing more, there is no way to know which it is.

Or if the molecular weight is 18 grams per mole, it could be water (H2O) or ammonium (NH4+). Again, no way to tell without some information about what elements the compound contains.

From the molecular formula, you can always determine the molar mass. See the Related Questions link to the left for how to do that.

Answer 3

To find the molecular weight (also called the molecular mass or molar mass) of a compound or

chemical, you need two things: the molecular formula of the compound and a Periodic Table.

The molecular formula tells you how many atoms of each element is present in the compound.

To find the molecular weight, just add up the atomic weights of each element present in the compound,

being sure to multiply by the number of times that atom appears. The atomic weight of each element

is found on a periodic table.

Answer 4

In order to find a substance's molecular formula, proceed with the following steps. An element's molecular formula is either going to be equal to the empirical formula or is a multiple of the empirical formula (n).

1. Find the empirical formula's molar mass. Calculate the molar mass of the empirical formula.

# atoms element A x Atomic Mass element A = mass A

# atoms element B x atomic mass element B = mass B

... etc.

Add up all of the mass values found above for the empirical formula's molar mass (MM).

2. Solve this equation (below).

n = MM compound (given) = Empirical Formula Units

---- MM emp. form.

3. After solving for n, multiply the empirical formula.

Molecular Formula = N (Empirical Formula)

Ex. What is the molecular formula of a compound which has an empirical formula of CH2 and a molar mass of 126.2g?

1. Find the empirical formula's molar mass.

1 atom carbon (C) x 12.01g = 12.01g

2 atoms hydrogen (H) = 2.016g

Empirical Molar Mass = 14.03

2. Solve this equation (below).

n = 126.2g substance = 9 Empirical Formula Units (n)

--------- 14.03g CH2

3. After solving for n, multiply the empirical formula.

Molecular Formula = 9 (CH2) --> C9H18

Answer 5

Molecular weight of unknown substanceDetermine the molecular weight of an unknown substance by measuring the freezing point depression of an aqueous solution of the unknown.

Answer 6

EMPIRICAL FORMULA

In order to find the empirical formula of a substance, proceed with the following steps:

1. Change the percent sign (%) of all elements to g (grams). No conversions for this step, just changing the sign to a g (percent to mass).

2. Convert mass to moles. This is done by dividing the mass by the substance's atomic mass (found on periodic table).

---- Mass

Atomic Mass

Conversion Factor

Mass x 1 mol element

---------- atomic mass

3. Divide the number of moles (found above) by the smallest number of moles in the compound. Do this for every element.

4. Multiply each number until whole. If the numbers are all whole or are equal to or less than 0.1 from a whole number, this step is not needed.

Ex. The percent composition of a compound is 25.94% Nitrogen (N), and 74.06% oxygen (O). Calculate the empirical formula for this substance.

1. Change the percent sign (%) of all elements to g (grams).

N = 25.94g

O = 74.06g

2. Convert mass to moles. Divide the mass of each element (step 1) by the element's atomic mass.

* Nitrogen mass --> mol (25.94g N/14.01g N)

---- 25.94g N x 1 mol N

------------------ 14.01g N = 1.852 mol N

* Oxygen mass --> mol (74.06g O/16.00g O)

---- 74.06g O x 1 mol O

----------------- 16.00g O = 4.629 mol C

3. Divide the number of moles (found above) by the smallest number of moles in the compound.

* N = 1.852 mol N = 1.000

------- 1.852 mol

* O = 4.629 mol O = 2.500

------- 4.629 mol

4. Multiply each number until whole. Since 2.500 is not within 0.1 away from being a whole number, multiply it until whole (in this case, multiply by 2).

N = 1.000 x 2 = 2

O = 2.500 x 2 = 5

Empirical Formula: N2O5 (Dinitrogen Pentoxide)

MOLECULAR FORMULA

In order to find a substance's molecular formula, proceed with the following steps. An element's molecular formula is either going to be equal to the empirical formula or is a multiple of the empirical formula (n).

1. Find the empirical formula's molar mass. Calculate the molar mass of the empirical formula.

# atoms element A x atomic mass element A = mass A

# atoms element B x atomic mass element B = mass B

... etc.

Add up all of the mass values found above for the empirical formula's molar mass (MM).

2. Solve this equation (below).

n = MM compound (given) = Empirical Formula Units

---- MM emp. form.

3. After solving for n, multiply the empirical formula.

Molecular Formula = N (Empirical Formula)

Ex. What is the molecular formula of a compound which has an empirical formula of CH2 and a molar mass of 126.2g?

1. Find the empirical formula's molar mass.

1 atom carbon (C) x 12.01g = 12.01g

2 atoms hydrogen (H) = 2.016g

Empirical Molar Mass = 14.03

2. Solve this equation (below).

n = 126.2g substance = 9 Empirical Formula Units (n)

--------- 14.03g CH2

3. After solving for n, multiply the empirical formula.

Molecular Formula = 9 (CH2) --> C9H18

Answer 7

To find the empirical formula, determine the simplest whole-number ratio of atoms in the compound. To find the molecular formula, compare the compound's molar mass to the empirical formula's molar mass to calculate the multiplier. Multiply the subscripts in the empirical formula by this multiplier to get the molecular formula.

Answer 8

This is possible only after a chemical analysis of this compound.