Atomic Mass Units
What does amu stand for?
Atomic Mass Units
What number does one mole represent?
6.022×1023
What is the percent by mass of oxygen in water, H2O?
(H = 1.0 g/mol, O = 16.0 g/mol)
88.9%
What is the empirical formula of C6H12O6?
CH2O
(All subscripts are divided by 6.)
Which of the following molecular formulas has a molar mass closest to 44.0 g/mol?
CO / CO2 / C2H4
CO2
12.0+2(16.0)=44.0 g/mol
What is the total mass, in amu, of a single carbon atom?
12 amu
How many moles are in 6.022×1023 atoms of an element?
1 mole
What is the percent by mass of carbon in carbon dioxide, CO2?
(C = 12.0 g/mol, O = 16.0 g/mol)
27.3%
What is an empirical formula?
The empirical formula shows the simplest whole-number ratio of atoms of each element in a compound.
A compound has the empirical formula CH2 and a molar mass of 42.0 g/mol. What is the molecular formula?
C3H6
Each helium atom has a mass of 4 amu. What is the total mass, in amu, of 5 helium atoms?
20 amu
How many atoms are in 2.0 moles of sodium?
1.20×1024 atoms
What does percent composition tell you about a compound?
It tells you the percentage by mass of each element in a compound.
A compound contains 24.0 g of carbon and 4.0 g of hydrogen. What is the empirical formula?
(C = 12.0 g/mol, H = 1.0 g/mol)
CH2
How is a molecular formula different from an empirical formula?
A molecular formula shows the actual number of each type of atom in a molecule, while an empirical formula shows the simplest whole-number ratio of those atoms.
A sample has a total mass of 48 amu and is made entirely of oxygen atoms. How many oxygen atoms are in the sample?
3 oxygen atoms
How many moles are in 48.0 g of carbon?
4 moles
A 50.0 g sample contains 20.0 g of sodium and the rest is chlorine. What is the percent composition of sodium in the sample?
40.0%
A compound contains 27.3% carbon and 72.7% oxygen by mass. What is the empirical formula?
(C = 12.0 g/mol, O = 16.0 g/mol)
CO2
A compound has the empirical formula NO2 and a molar mass of 92.0 g/mol. What is the molecular formula?
N2O4
Calculate the mass, in amu, of a sample of aluminum that contains 75 atoms.
2023.5 amu
One mole of water (H2O) contains how many moles of hydrogen atoms?
2 moles of hydrogen atoms
Nitric oxide (NO) and nitrogen dioxide (NO2) both contain nitrogen and oxygen. Which one has a higher percentage of nitrogen by mass? Explain.
NO.
Adding more oxygen increases the total mass without increasing the nitrogen mass, so nitrogen makes up a smaller percent in NO₂ than in NO.
A compound is 52.2% carbon and 47.8% oxygen by mass. What is the empirical formula of the compound?
(C = 12.0 g/mol, O = 16.0 g/mol)
C3O2
A compound’s empirical formula is C2H4O. The compound’s molar mass is 220. g/mol. What is the molecular formula?
C10H20O5