4th TERM TEST

What is the empirical formula for C₄H₆?

• CH

• CH₃

• C₂H₃

• C₄H₆

What is the empirical formula for N₂S₃?

• NS

• N₃S₂

• N₂S₃

 What is the molecular mass of HO (in grams)?

A formula with the lowest whole # molar ratio of elements in a compound is called

• Molecular Formula
• Chemical Formula
• Empirical Formula 

You are given the following percentages: 40.05% S and 59.95% O. Find the empirical formula for these elements.

• SO
• SO2
• SO3
• SO4

Answer explanation

When you are given %'s and asked for the formula, you need moles!

1) write the %'s as grams (here 40.05 g S and 59.95 g O)

2) use the molar masses from the Per. Tab. to convert grams -> moles.

3) if your moles are not whole numbers, you should either double all of the answers (if one of them ends in 0.5) OR divide all of the answers by the smallest answer.

** Remember, you are looking for whole number mole ratios to make a formula!

Which one is an empirical formula?

• H₂O₂
• C₂H₆O₁₂
• CaCl₂
• N₂O₈

Answer explanation

An empirical formula is the lowest whole number ratio. Which one of the formula was the most reduced/ simplified version?

Glycerol has a molecular molar mass of 92.09g/mol. The empirical formula is C₃H₈O₃. What is the molecular formula for glycerol?

• C₂H₃O₂
• CH₂O
• C₂H₄O₂
• C₃H₈O₃

Answer explanation

You have the empirical formula! What is the molar mass of this formula?? Compare this to the mass in the problem! They are equal! So the formulas are equal!

The Empirical Formula for a substance is CF₂. The Molecular formula mass is 200. g/mol. What is the Molecular formula?

• C₄F₈
• C₄F
• CF₈
• C₂F₄

Answer explanation

Here you are given 2 pieces of information:

1) the empirical formula.

2) the mass of the molecular formula.

To see if your empirical = molecular, you need to calculate the molar mass of the empirical formula!

If this mass is smaller than the mass the problem gives you, take the BIGGER MASS and divide by the smaller one! This tells you what you need to multiply your current formula by.

What is the empirical formula if you have 81.82% carbon and 18.18% hydrogen?

• C3H8

• CH4

• C2H2

• C4H10

Glycerol has a molar mass of 92.09g/mol. Its percent composition is: 39.12% C, 8.75% H, and 51.12% O. What is the molecular formula for glycerol?

• C₂H₃O₂
• CH₂O
• C₂H₄O₂
• C₃H₈O₃