Formula Weight + Percent Composition

When finding formula weight, what do you add together?

The atomic masses of every atom in the formula

Where do you get atomic masses from during the exam?

The periodic table

If a formula has a subscript, what should you do with the atomic mass?

Multiply the atomic mass by the subscript

If a formula has parentheses with a subscript outside, what should you do?

Multiply everything inside the parentheses by the outside subscript

What does formula weight use as its unit?

amu

What formula should you use for percent by mass?

Mass of the element in the compound ÷ total formula mass × 100

In percent composition, what goes on the bottom of the fraction?

Total formula mass of the compound

In percent composition, what goes on the top of the fraction?

Total mass of the element being asked about

How many oxygen atoms are in Pb(NO₃)₂?

6 oxygen atoms

How many nitrogen atoms are in Pb(NO₃)₂?

2 nitrogen atoms

How many lead atoms are in Pb(NO₃)₂?

1 lead atom

What is the formula weight of Pb(NO₃)₂?

331.22 amu

What is the percent by mass of oxygen in Pb(NO₃)₂?

29.0%

Why is oxygen in Pb(NO₃)₂ counted as 6 oxygen atoms?

There are two nitrate groups, and each nitrate has 3 oxygen atoms

How many potassium atoms are in K₂Cr₂O₇?

2 potassium atoms

How many chromium atoms are in K₂Cr₂O₇?

2 chromium atoms

How many oxygen atoms are in K₂Cr₂O₇?

7 oxygen atoms

What is the formula weight of K₂Cr₂O₇?

294.18 amu

What is the percent by mass of oxygen in K₂Cr₂O₇?

38.1%

What is the percent by mass of chromium in K₂Cr₂O₇?

35.4%

How many hydrogen atoms are in H₂O?

2 hydrogen atoms

How many oxygen atoms are in H₂O?

1 oxygen atom

What is the formula weight of H₂O?

18.02 amu

What is the percent by mass of hydrogen in H₂O?

11.2%

What is the percent by mass of oxygen in H₂O?

88.8%

Why is oxygen’s percent high in H₂O even though there is only one oxygen atom?

Oxygen has much more mass than hydrogen

How many carbon atoms are in CO₂?

1 carbon atom

How many oxygen atoms are in CO₂?

2 oxygen atoms

What is the formula weight of CO₂?

44.01 amu

What is the percent by mass of carbon in CO₂?

27.3%

What is the percent by mass of oxygen in CO₂?

72.7%

How many atoms total are in CO₂?

3 atoms

How many atoms total are in H₂O?

3 atoms

Does total number of atoms always tell you which element has the highest percent mass?

No

Why does atom count alone not determine percent mass?

Elements have different atomic masses

How many calcium atoms are in CaCl₂?

1 calcium atom

How many chlorine atoms are in CaCl₂?

2 chlorine atoms

What is the formula weight of CaCl₂?

110.98 amu

What is the percent by mass of calcium in CaCl₂?

36.1%

What is the percent by mass of chlorine in CaCl₂?

63.9%

How many magnesium atoms are in MgO?

1 magnesium atom

How many oxygen atoms are in MgO?

1 oxygen atom

What is the formula weight of MgO?

40.31 amu

What is the percent by mass of magnesium in MgO?

60.3%

What is the percent by mass of oxygen in MgO?

39.7%

How many aluminum atoms are in Al₂O₃?

2 aluminum atoms

How many oxygen atoms are in Al₂O₃?

3 oxygen atoms

What is the formula weight of Al₂O₃?

101.96 amu

What is the percent by mass of aluminum in Al₂O₃?

52.9%

What is the percent by mass of oxygen in Al₂O₃?

47.1%

How many magnesium atoms are in Mg(NO₃)₂?

1 magnesium atom

How many nitrogen atoms are in Mg(NO₃)₂?

2 nitrogen atoms

How many oxygen atoms are in Mg(NO₃)₂?

6 oxygen atoms

What is the formula weight of Mg(NO₃)₂?

148.33 amu

What is the percent by mass of nitrogen in Mg(NO₃)₂?

18.9%

What is the percent by mass of oxygen in Mg(NO₃)₂?

64.7%

How many calcium atoms are in CaCO₃?

1 calcium atom

How many carbon atoms are in CaCO₃?

1 carbon atom

How many oxygen atoms are in CaCO₃?

3 oxygen atoms

What is the formula weight of CaCO₃?

100.09 amu

What is the percent by mass of calcium in CaCO₃?

40.0%

What is the percent by mass of oxygen in CaCO₃?

48.0%

How many aluminum atoms are in Al₂(SO₄)₃?

2 aluminum atoms

How many sulfur atoms are in Al₂(SO₄)₃?

3 sulfur atoms

How many oxygen atoms are in Al₂(SO₄)₃?

12 oxygen atoms

What is the formula weight of Al₂(SO₄)₃?

342.14 amu

What is the percent by mass of oxygen in Al₂(SO₄)₃?

56.1%

How many carbon atoms are in C₆H₁₂O₆?

6 carbon atoms

How many hydrogen atoms are in C₆H₁₂O₆?

12 hydrogen atoms

How many oxygen atoms are in C₆H₁₂O₆?

6 oxygen atoms

What is the formula weight of C₆H₁₂O₆?

180.16 amu

What is the percent by mass of carbon in C₆H₁₂O₆?

40.0%

What is the percent by mass of hydrogen in C₆H₁₂O₆?

6.7%

What is the percent by mass of oxygen in C₆H₁₂O₆?

53.3%

How many carbon atoms are in octane, C₈H₁₈?

8 carbon atoms

How many hydrogen atoms are in octane, C₈H₁₈?

18 hydrogen atoms

What is the formula weight of C₈H₁₈?

114.22 amu

What is the percent by mass of carbon in C₈H₁₈?

84.1%

What is the percent by mass of hydrogen in C₈H₁₈?

15.9%

How many iron atoms are in FeBr₃?

1 iron atom

How many bromine atoms are in FeBr₃?

3 bromine atoms

What is the formula weight of FeBr₃?

295.55 amu

What is the percent by mass of bromine in FeBr₃?

81.1%

How many barium atoms are in Ba₃(PO₄)₂?

3 barium atoms

How many phosphorus atoms are in Ba₃(PO₄)₂?

2 phosphorus atoms

How many oxygen atoms are in Ba₃(PO₄)₂?

8 oxygen atoms

What is the formula weight of Ba₃(PO₄)₂?

601.93 amu

What is the percent by mass of oxygen in Ba₃(PO₄)₂?

21.3%

If the problem asks for percent oxygen, should you include every oxygen atom in the formula?

Yes

If the problem asks for percent nitrogen in Mg(NO₃)₂, how many nitrogen atoms should you count?

2 nitrogen atoms

If the problem asks for percent oxygen in Al₂(SO₄)₃, how many oxygen atoms should you count?

12 oxygen atoms

What is the most common mistake in percent composition problems with parentheses?

Forgetting to multiply the atoms inside parentheses

What should your percent compositions add up to if you calculate every element in a compound?

100%

If your percent composition is greater than 100%, what probably happened?

The setup or total formula mass is wrong

If you only use one oxygen for Pb(NO₃)₂, what mistake did you make?

You ignored the subscripts and parentheses