ATOMS AND MOLECULES

Write the formulae for the following and calculate the molecular mass for
each one of them.
(a) Caustic potash
(b) Baking powder
(c) Lime stone
(d) Caustic soda
(e) Ethanol
(f) Common salt

(a) In a chemical reaction, the sum of the masses of the reactants and
products remains unchanged. This is called ————.
(b) A group of atoms carrying a fixed charge on them is called ————.
(c) The formula unit mass of Ca3 (PO4)2 is ————.
(d) Formula of sodium carbonate is ———— and that of ammonium
sulphate is ————.

A silver ornament of mass ‘m’ gram is polished with gold equivalent to 1%
of the mass of silver. Compute the ratio of the number of atoms of gold and
silver in the ornament.

Compute the difference in masses of one mole each of aluminium atoms
and one mole of its ions. (Mass of an electron is 9.1×10–28 g). Which one is
heavier?

What are ionic and molecular compounds? Give examples.

A gold sample contains 90% of gold and the rest copper. How many atoms
of gold are present in one gram of this sample of gold?

Compute the number of ions present in 5.85 g of sodium chloride.

Raunak took 5 moles of carbon atoms in a container and Krish also took 5
moles of sodium atoms in another container of same weight. (a) Whose
container is heavier? (b) Whose container has more number of atoms?

Cinnabar (HgS) is a prominent ore of mercury. How many grams of mercury
are present in 225 g of pure HgS? Molar mass of Hg and S are
200.6 g mol–1 and 32 g mol–1 respectively.

The difference in the mass of 100 moles each of sodium atoms and sodium
ions is 5.48002 g. Compute the mass of an electron.

Calculate the number of moles of magnesium present in a magnesium
ribbon weighing 12 g. Molar atomic mass of magnesium is 24g mol–1.

You are provided with a fine white coloured powder which is either sugar
or salt. How would you identify it without tasting?

Does the solubility of a substance change with temperature? Explain with
the help of an example.

What is the fraction of the mass of water due to neutrons?

Give the chemical formulae for the following compounds and compute the
ratio by mass of the combining elements in each one of them. (You may
use appendix-III).
(a) Ammonia
(b) Carbon monoxide
(c) Hydrogen chloride
(d) Aluminium fluoride
(e) Magnesium sulphide

Which of the following symbols of elements are incorrect? Give their correct
symbols
(a) Cobalt                  CO
(b) Carbon                 c
(c) Aluminium            AL
(d) Helium                 He
(e) Sodium                So

Give the formulae of the compounds formed from the following sets of
elements
(a) Calcium and fluorine
(b) Hydrogen and sulphur
(c) Nitrogen and hydrogen
(d) Carbon and chlorine
(e) Sodium and oxygen
(f) Carbon and oxygen

Write the molecular formulae for the following compounds
(a) Copper (II) bromide
(b) Aluminium (III) nitrate
(c) Calcium (II) phosphate
(d) Iron (III) sulphide
(e) Mercury (II) chloride
(f) Magnesium (II) acetate

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)

Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.

What is the mass of:

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

Convert into mole.

(a) 12 g of oxygen gas

(b) 20 g of water

(c) 22 g of carbon dioxide

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 (Na2SO3)?

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

Give the names of the elements present in the following compounds:

(a) Quick lime

(b) Hydrogen bromide

(c) Baking powder

(d) Potassium sulphate

Write the chemical formulae of the following:

(a) Magnesium chloride

(b) Calcium oxide

(c) Copper nitrate

(d) Aluminium chloride

(e) Calcium carbonate

What are polyatomic ions? Give examples?

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 combinations will govern your answer?

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.

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)

If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?

 Calculate the formula unit masses of ZnO, Na2O ,K2co3 given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Calculate the molecular masses of H2, O2, Cl2, CO2, CH4, C2H6, C2H4, NH3, CH3OH.

How many atoms are present in a

(i) H2S molecule and

(ii) PO43-ion?

What is meant by the term chemical formula?

Write down the names of compounds represented by the following formulae:

(i) Al2(SO4)3

(ii) CaCl2

(iii) K2SO4

(iv) KNO3

(v) CaCO3

Write down the formulae of

(i) sodium oxide

(ii) aluminium chloride

(iii) sodium sulphide

(iv) magnesium hydroxide

Why is it not possible to see an atom with naked eyes?

Define atomic mass unit.

Which postulate of Dalton's atomic theory can explain the law of definite proportions?

Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?

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?

In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products were 2.2 g of carbon dioxide, 0.9 g water and 8.2 g of sodium ethanoate. Show that these observations are in agreement with the law of conservation of mass.

Sodium carbonate + ethanoic acid → sodium ethanoate + carbon dioxide + water