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Answers additional questions 7.1
1.
Describe how the particles of sodium bromide, an ionic compound, are different from water, a covalent
compound. Give the particle name of each compound type.
Ionic compounds are made of cations (+) and anions (-) held together by electrostatic attraction in a
regular crystal structure. The ionic bond is caused by a transfer of electrons. The smallest ratio of cations to
anions is a formula unit.
Covalent compounds are atoms held by a sharing of their outer electrons. The group of covalently
bonded atoms is called a molecule.
2.
The acme fish lure company makes small, medium and large lures. The small lures have a mass of 8.5
grams, the medium lures 12.9 grams and the large lures 22.0 grams.
A.
The company makes a starter kit with 3 large lures, 5 medium lures, and 5 small lures. If the packaging
has a mass of 5.5 grams, what is the total mass of the starting kit?
3 large lures x 22.0 g/lure + 5 medium lures x 12.9 g/lure + 5 small lures x 8.5 g/lure + 5.5 g packaging
= 178.5 g
To find the mass of a compound first write the formula of the compound and then add the # atoms of each
element x mass of 1 atom of the element for all the elements in the compound
B.
The supply company just sent a acme box of 25 of the same size lures.
If the box had a mass of 323 grams, what size lures did it contain?
323g total / 25 lures = 12.92 g/lure
The lures were medium size
You can count how many of something of you have if you know the total mass and the mass of an individual
particle.
C.
Acme is marketing an economy pack of 10 of each lure size.
What mass of large, medium, and small lures should be used for each pack?
10 large lures * 22.0 g/lure + 10 medium lures * 12.9 g/lure + 10 small lures * 8.5 g/lure = 434 grams
The total mass of a compound is the sum of the masses of each element that make it up.
3.
What is the average mass of a sulfur atom?
32.066 atomic mass units.
The mass on the periodic table is a weighed average of the isotopes of the element. sum of (% of each isotope *
mass of each isotope)
What units are subatomic particles measured in?
1 proton and 1 neutron are each about 1 atomic mass unit.
The electron is about 1 /1823 amu.
The relative mass of atoms is based the carbon 12 atom with 6 electrons, 6 protons, and 6 neutrons.
How many electrons, neutrons, and protons does the sulfur atom contain?
The atomic number of sulfur is 16, meaning it has 16 electrons and 16 protons when unionized.
Sulfur has several isotopes with the same atomic number but different atomic masses.
Sulfur 32 would have 16 neutrons because the Atomic mass - atomic number = # neutrons.
4.
What is the mass of 1 formula unit of magnesium nitrate?
List the number of atoms of each element in 1 formula unit of magnesium nitrate.
Mg(NO3 )2
1 atom Mg,
2 atoms N,
6 atoms O
1 * 24.3 amu + 2 * 14.0 + 6 * 16.0 = 148.3 amu
This is the formula mass of an ionic compound
5.
Table sugar is sucrose (C12H22O11), a covalent compound.
What mass does a sucrose molecule have?
12 carbon * 12.0 + 22 hydrogen * 1.0 + 11 oxygen * 16.0 = 342 amu
This is the molecular mass of a covalent compound
6.
What is the mass of a single carbon atom in amu?
1 carbon atom = 12.0 atomic mass units
What mass would a mole of carbon atoms have?
Gram atomic mass= the mass of 1 atom of an element expressed in grams = mass of 1 mole of a element
= 12.0 grams for 1 mole of carbon
How many carbon atoms are in a mole of carbon?
One mole of anything = 6.02 x 1023 particles
7.
Which would have more atoms, 5 grams of iron or 5 grams of magnesium? Why?
1 mole of iron = 6.02 x 1023 atoms of iron = 55.8 grams
1 mole of magnesium = 6.02 x 1023 atoms of magnesium = 24.3 grams
Each atom of magnesium has a smaller mass than an atom of iron.
5.0 grams Fe =
55.8 grams Fe
X atoms Fe
6.02 x 1023 atoms Fe
X= 5.4 x 1022 atoms Fe
5.0 grams Mg =
24.3 grams Mg
Y atoms Mg
6.02 x1023 atoms Mg
Y=1.2 x 1023 atoms Mg
Because the magnesium atoms are lighter than the iron atoms there are more magnesium atoms per unit mass
8.
What mass units are used to measure moles?
particles (atoms, molecules, formula units) are measured
in atomic mass units
moles (elements, covalent compounds, ionic compounds)
are measured in grams
What is the mass of 1 mole of carbon dioxide?
CO2
9.
H2 O
1 mole C * 12.0 g + 2 moles O * 16.0 g = 44.0 g
How moles of water are in 3.6 grams of water?
2 moles H * 1.0 g + 1 mole O * 16.0 g = 18.0 g/mole
3.6 g H2 O * 1 mole H2 O / 18.0 g = 0.20 moles H2 O
10.
How many grams are in 1.3 moles of aluminum sulfide?
Al2 S3 2 Al * 27.0 g/mole + 3 S * 32.0 g/mole = 150 g/mole
1.3 moles Al2 S3 * 150 g / 1 mole Al2 S3 = 195 grams Al2 S3
11.
How many moles of phosphorus are in 1 mole of calcium phosphate?
1 formula unit of calcium phosphate Ca3 (PO4 )2 has
3 calcium atoms, 2 phosphorus atoms, 8 oxygen atoms
1 mole of calcium phosphate has
3 moles of calcium, 2 moles of phosphorus, 8 moles of oxygen
How many grams of phosphorus are in 1 mole of calcium phosphate?
1 mole Ca3 (PO4 )2 * 2 mole P / 1 mole Ca3 (PO4 )2 * 31.0 g / 1 mole P
= 62.0 g P
What percent of the total mass of calcium phosphate is phosphorus?
2 mole P = 62.0 g
3 mole Ca * 40.1 g + 2 mole P * 31.0 g + 8 mole O * 16.0 g
=
62.0 g P
310.3 g total
=
20.0 % P by mass
12.
Find the percentage of magnesium, oxygen, and hydrogen by mass in magnesium hydroxide.
Mg(OH)2
1 mole Mg = 24.3 g
Total mass = 58.3 g
2 moles O = 32.0 g
% Mg = 24.3 g / 58.3 g =41.7%
% H = 2.0 g/ 58.3 g = 3.4%
2 moles H = 2.0 g
% O = 32.0 g / 58.3 g = 54.9%
13.
Red marbles have a mass of 3 grams and blue marbles have a mass of 6 grams.
A box has twice as many red marbles as blue marbles.
red = 2 * blue
If the total mass of marbles is 60 grams, how many of each color do you have?
red * 3 g + blue * 6 g = 60 g
Substitute the first equation in the second equation
(2 * blue) * 3 g + blue * 6 g = 60g
6 blue + 6 blue = 60
12 blue = 60
blue = 5 marbles
Use the first equation to get the red marbles from the # blue marbles
red = 2 * 5 or 10 marbles
5 blue marbles * 6g + 10 red marbles* 3g = 60g
If you know the mass of each element in a compound and you know the total mass of the compound you can
find the formula.
Think of the red marbles as atoms of 1 element and the blue marbles as atoms of another element.
We knew the ratio of red to blue particles was 2:1
We knew the total mass was 60
We calculated the formula to be B 5 R10
14.
A compound is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass.
What is the empirical formula of the compound?
Divide the percentage of each element by the mass of 1 atom of the element to convert from a percentage by
mass to a ratio of particles
C
H
O
40.0% / 12.0 amu =3.3333
6.7% / 1.0 amu = 6.7
53.3% / 16.0 amu = 3.33125
{These are ratios of particles}
Divide each particle ratio by the smallest number to get whole numbers of atoms
C
H
O
3.333 / 3.333 = 1 atom C
6.7 / 3.33 = 2 atoms H
3.33125 / 3.333 = 1 atom O
The simplest ratio of atoms is the empirical formula CH2 O
If the compound has a molecular mass of 180 amu, what is the molecular formula of the compound?
The molecular formula must be a whole number multiple of the empirical formula.
The mass of the empirical formula is
1 C * 12.0 + 2 H * 1.0 + 1 O * 16.0 = 30 amu
180 amu / 30 amu = 6 The molecular formula must be 6 times the empirical formula or C6 H12O6
15.
The equation is balanced to produce the same number of atoms of each element in the reactants and
products.
Reactants
1 molecule N2 x 2 N atoms/ 1 N2 molecule = 2 N atoms
3 molecules H2 x 2 H atoms / 1 H2 molecule = 6 H atoms
Products
2 molecules NH3 x 1 N atom / 1 NH3 molecule = 2 N atoms
2 molecules NH3 x 3 H atoms / 1 NH3 molecule = 6 H atoms
The total number of moles of atoms of each element stays the same.
The number of moles of molecules varies with the number of atoms in each compound molecule.