# What does "formula weight (equivalent weight)" mean?

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#### chiddler

##### Full Member
10+ Year Member

Or relative molar mass.

"Molecular weight (M.W.) and formula weight (F.W.) are older terms for what is now more correctly called the relative molar mass (Mr). This is a dimensionless quantity (i.e., a pure number, without units) equal to the molar mass divided by the molar mass constant."

Berkeley review says that it is empirical weight. I want to confirm because confusing wiki definition. thanks.

##### snow, PBR, and bears
10+ Year Member
Wiki is just trying to say that technically there are no units on the numbers you pull off of the periodic table. So if you add up the numbers for water and get 18.02, you have to multiply by the "molar mass constant" (defined as exactly 1 g/mol) to get the value you were looking for, 18.02g/mol.

Pretty stupid and not relevant to the mcat if you ask me.

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#### chiddler

##### Full Member
10+ Year Member
Wiki is just trying to say that technically there are no units on the numbers you pull off of the periodic table. So if you add up the numbers for water and get 18.02, you have to multiply by the "molar mass constant" (defined as exactly 1 g/mol) to get the value you were looking for, 18.02g/mol.

Pretty stupid and not relevant to the mcat if you ask me.

was part of a question, but you're probably right.

you divided by 1 g/mol because H2O cannot be reduced to anything simpler, right? If H2O2, then it's divided by 2 making it 17.

##### snow, PBR, and bears
10+ Year Member
The molar mass of H2O2 is 34.02g/mol. Divide it by the "molar mass constant" (defined to be 1g/mol) to get the molar weight / molecular weight / relative molar mass of 34.02. Again this is just wiki being pedantic and not mcat worthy.

The empirical weight is the weight of the empirical formula. The empirical formula is the molecular formula reduced to its smallest common multiple. So H2O2's empirical formula is OH, and its empirical weight is 17.01g/mol. Glucose is C6H12O6, empirical formula is CH2O (divide everything by 6), and empirical weight is 30.02g/mol.

#### chiddler

##### Full Member
10+ Year Member
The molar mass of H2O2 is 34.02g/mol. Divide it by the "molar mass constant" (defined to be 1g/mol) to get the molar weight / molecular weight / relative molar mass of 34.02. Again this is just wiki being pedantic and not mcat worthy.

The empirical weight is the weight of the empirical formula. The empirical formula is the molecular formula reduced to its smallest common multiple. So H2O2's empirical formula is OH, and its empirical weight is 17.01g/mol. Glucose is C6H12O6, empirical formula is CH2O (divide everything by 6), and empirical weight is 30.02g/mol.

ha. that is pretty useless.

#### MedPR

##### Membership Revoked
Removed
There was a question about molecular weight vs formula weight in a passage about polyprotic acids. That was pretty dumb also.

#### drechie

##### Full Member
10+ Year Member
The Berkeley Review:

What is the formula weight (equivalent weight) of oxalic acid (HO2CCO2H)?

A. B. C. D.

45gramspermole 90 grams per mole

135 grams per mole 180 grams per mole

answer was 45 grams per mole... why

1. Choice A is correct. The formula weight, also known as the empirical weight, is derived from the mass of compound per mole of equivalent. Adding up the molecular mass of the compound yields 90 grams/mole. Because there are two equivalents of protons per oxalic acid, the formula weight is 45 grams/mole. Pick A. Be sure not to pick B by mistake.

#### IslandStyle808

##### Akuma residency or bust!
7+ Year Member
The Berkeley Review:

What is the formula weight (equivalent weight) of oxalic acid (HO2CCO2H)?

A. B. C. D.

45gramspermole 90 grams per mole

135 grams per mole 180 grams per mole

answer was 45 grams per mole... why

1. Choice A is correct. The formula weight, also known as the empirical weight, is derived from the mass of compound per mole of equivalent. Adding up the molecular mass of the compound yields 90 grams/mole. Because there are two equivalents of protons per oxalic acid, the formula weight is 45 grams/mole. Pick A. Be sure not to pick B by mistake.

Hey there,

What they are giving now is the molecular formula (HO2CCO2H), so the molar weight is 90 grams per mole. However, you want the empirical weight (formula weight), so you just basically divide the C's, H's, and O's by 2. This gives you the empirical formula CO2H which has a empirical weight of 45 grams per mole.