Molecular and Formula Masses
Atomic mass of Chlorine is 35.453 u. Chlorine (17 Cl) has 25 isotopes with mass numbers ranging from 28 Cl to 52 Cl and 2 isomers (34m Cl and 38m Cl). There are two stable isotopes, 35 Cl (75.77%) and 37 Cl (24.23%), giving chlorine a standard atomic weight of 35.45. The longest-lived radioactive isotope is 36 Cl, which has a half-life of 301,000 years. All other isotopes have half-lives under 1 hour, many less than one second.
The molecular mass of a substance is the sum of the average masses of the atoms in one molecule of a substance. It is calculated by adding together the atomic masses of the elements in the substance, each multiplied by its subscript (written or implied) in the molecular formula. Because the units of atomic mass are atomic mass units, the units of molecular mass are also atomic mass units. The procedure for calculating molecular masses is illustrated in Example (PageIndex{1}).
Example (PageIndex{1}): Ethanol
The total mass of the products is 50g. Which best completes the other two amounts? The amount of CD is 40 g, and the amount of AC is 35 g. The amount of CD is 35 g, and the amount of AC is 40 g. The amount of CD and AC would be the same. The amount of CD and AC is undetermined.
Calculate the molecular mass of ethanol, whose condensed structural formula is (ce{CH_3CH_2OH}). Among its many uses, ethanol is a fuel for internal combustion engines
Solution
| Steps for Problem Solving | Calculate the molecular mass of ethanol, whose condensed structural formula is (ce{CH_3CH_2OH}) |
|---|---|
| Identify the 'given'information and what the problem is asking you to 'find.' | Given: Ethanol molecule (CH3CH2OH) Find: molecular mass |
Determine the number of atoms of each element in the molecule. | The molecular formula of ethanol may be written in three different ways:
All show that ethanol has two carbon atoms, six hydrogen atoms, and one oxygen atom. |
Obtain the atomic masses of each element from the periodic table and multiply the atomic mass of each element by the number of atoms of that element. | 1 C atom = 12.011 amu 1 H atom = 1.0079 amu 1 O atom = 15.9994 amu |
| Add the masses together to obtain the molecular mass. | 2C: (2 atoms)(12.011amu/atom) = 24.022 amu 6H: (6 atoms)(1.0079amu/atom) = 6.0474amu +1O: (1 atoms)(15.9994amu/atom) =15.9994amu C2H6O : molecular mass of ethanol = 46.069amu |
-+the+total+number+of+atoms+in+a+molecular+compound.+Cl2+%3D+2+x+35.4+g+%3D+70.8+g..jpg "Mass")
Exercise (PageIndex{1}): Freon
Calculate the molecular mass of trichlorofluoromethane, also known as Freon-11, which has a condensed structural formula of (ce{CCl3F}). Until recently, it was used as a refrigerant. The structure of a molecule of Freon-11 is as follows:
- Answer
- 137.37 amu
Unlike molecules, which form covalent bonds, ionic compounds do not have a readily identifiable molecular unit. Therefore, for ionic compounds, the formula mass (also called the empirical formula mass) of the compound is used instead of the molecular mass. The formula mass is the sum of the atomic masses of all the elements in the empirical formula, each multiplied by its subscript (written or implied). It is directly analogous to the molecular mass of a covalent compound. The units are atomic mass units.
Atomic mass, molecular mass, and formula mass all have the same units: atomic mass units.
Example (PageIndex{2}): Calcium Phosphate
Calculate the formula mass of (ce{Ca3(PO4)2}), commonly called calcium phosphate. This compound is the principal source of calcium found in bovine milk.
Solution
| Steps for Problem Solving | Calculate the formula mass of (ce{Ca3(PO4)2}), commonly called calcium phosphate. |
|---|---|
| Identify the 'given' information and what the problem is asking you to 'find.' | Given: Calcium phosphate [Ca3(PO4)2] formula unit Find: formula mass |
Determine the number of atoms of each element in the molecule. |
|
Obtain the atomic masses of each element from the periodic table and multiply the atomic mass of each element by the number of atoms of that element. | 1 Ca atom = 40.078 amu 1 P atom = 30.973761 amu 1 O atom = 15.9994 amu |
| Add together the masses to give the formula mass. | 3Ca: (3 atoms) (40.078 amu/atom)=120.234amu 2P: (2 atoms) (30.973761amu/atom)=61.947522amu + 8O: (8 atoms)(15.9994amu/atom)=127.9952amu Formula mass of Ca3(PO4)2=310.177amu |
Atomic Mass Of Cl2o7
Exercise (PageIndex{2}): Silicon Nitride
Calculate the formula mass of (ce{Si3N4}), commonly called silicon nitride. It is an extremely hard and inert material that is used to make cutting tools for machining hard metal alloys.
- Answer
- 140.29 amu
Cl2 Molar Mass
Page 32Page 32
Question 2. Hydrogen and oxygen combine in the ratio of 1 : 8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Hydrogen : Oxygen —> H2O
∴ 1 : 8 = 3 : x
x = 8 x 3
x = 24 g
∴ 24 g of oxygen gas would be required to react completely with 3 g of hydrogen gas.
Question 3. Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Question 4. Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Page 35
Question 2. Why is it not possible to see an atom with naked eyes?
1 m = 109 nm
Page 39
Question 1. Write down the formulae of
(i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide
Question 2. What is meant by the term chemical formula?
Question 3. How many atoms are present in a
(i) H2S molecule and
(ii) P043- ion?
(ii) P043- —> 5 atoms are present
Page 40
Question 2.Calculate the formula unit masses of ZnO, Na2O, K2C03, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
(i) ZnO = 65 u + 16 u = 81 u
(ii) Na2O = (23 u x 2) + 16 u = 46 u + 16 u = 62 u
(iii) K2C03 = (39 u x 2) + 12 u + 16 u x 3
= 78 u + 12 u + 48 u = 138 u
Page 42
Question 1. If one mole of carbon atoms weigh 12 grams, what is the mass (in grams) of 1 atom of carbon?Answer:
Question 2. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given atomic mass of Na = 23 u, Fe = 56 u)?Answer:
Exercise
Question 1. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.Answer: Boron and oxygen compound —> Boron + Oxygen
0.24 g —> 0.096 g + 0.144 g
Question 2. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical combination will govern your answer?
It shows that 12 g of carbon bums in 32 g oxygen to form 44 g of carbon dioxide. Therefore 3 g of carbon reacts with 8 g of oxygen to form 11 g of carbon dioxide. It is given that 3.0 g of carbon is burnt with 8 g of oxygen to produce 11.0 g of CO2. Consequently 11.0 g of carbon dioxide will be formed when 3.0 g of C is burnt in 50 g of oxygen consuming 8 g of oxygen, leaving behind 50 – 8 = 42 g of O2. The answer governs the law of constant proportion.
Question 3. What are poly atomic ions? Give examples.
Question 4. Write the chemical formulae of the following:
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Symbol —> Mg Cl
Change —> +2 -1
Formula —> MgCl2
(b) Calcium oxide
Symbol —> Ca O
Charge —> +2 -2
Formula —> CaO
(c) Copper nitrate
Symbol —> Cu NO
Change +2 -1
Formula -4 CU(N03)2
(d) Aluminium chloride
Symbol —> Al Cl
Change —> +3 -1
Formula —> AlCl3
(d) Calcium carbonate
Symbol —> Ca CO3
Change —> +2 -2
Formula —> CaC03
Question 5. Give the names of the elements present in the following compounds:
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Elements —> Calcium and oxygen
(b) Hydrogen bromide
Elements —> Hydrogen and bromine
(c) Baking powder —> Sodium hydrogen carbonate
Elements —> Sodium, hydrogen, carbon and oxygen
(d) Potassium sulphate
Elements —> Potassium, sulphur and oxygen
Question 6. Calculate the molar mass of the following substances.
(a) Ethyne, C2H2
(b) Sulphur molecule, S8
(c) Phosphorus molecule, P4 (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO3
(a) Ethyne, C2H2 = 2 x 12 + 2 x 1 = 24 + 2 = 26 g
(b) Sulphur molecule, S8 = 8 x 32 = 256 g
(c) Phosphorus molecule, P4=4 x 31 = i24g
(d) Hydrochloric acid, HCl = 1 x 1 + 1 x 35.5 = 1 + 35.5 = 36.5 g
(e) Nitric acid, HN03 = 1 x 1 + 1 x 14 + 3 x 16 = 1 + 14 + 48 = 63 g
Question 7. What is the mass of
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na2S03)?
(b) 4 moles of aluminium atoms
Mass of 1 mole of aluminium atoms = 27 g
∴ Mass of 4 moles of aluminium atoms = 27 x 4 = 108 g
(c) 10 moles of sodium sulphite (Na2SO3)
Mass of 1 mole of Na2SO3 = 2 x 23 + 32 + 3 x 16 = 46 + 32 + 48 = 126 g
∴ Mass of 10 moles of Na2SO3 = 126 x 10 = 1260 g
Question 8. Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of Carbon dioxide.
Molar mass of oxygen gas (O2) = 32 g
Mole of oxygen gas 12/32 = 0.375 mole
(b) Given mass of water = 20 g
Molar mass of water (H2O) = (2 x 1) + 16 = 18 g
Mole of water = 20/18 = 1.12 mole
(c) Given mass of Carbon dioxide = 22 g
Molar mass of carbon dioxide (CO2) = (1 x 12) + (2 x 16)
= 12 + 32 = 44 g
∴ Mole of carbon dioxide = 22/44 = 0.5 mole
Question 9. What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Molar mass of oxygen atoms = 16 g
Mass of oxygen atoms = 16 x 0.2 = 3.2 g
(b) Mole of water molecule = 0.5 mole
Molar mass of water molecules = 2 x 1 + 16= 18 g .
Mass of H2O = 18 x 0.5 = 9 g
Question 10. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
Given mass of sulphur = 16 g
Question 11. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide. (Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
= (2 x 27) + (3 x 16)
= 54 + 48 = 102 g.