Question:
Calculate the number of molecules of phosphorus (P4) present in 31 gram of phosphorus.
Answer:
Atomic mass of phosphorus = 31 u
Molecular mass of P4 = 31× 4 = 124 u
⇒ Molar mass of P4 = 124 g/mol.
⇒ Mass of 1 mole ( 6.023 × 1023 ) P4 molecules = 124 g
∴ 124 g of P4 = 6.023 × 1023 molecules of P4
1g of P4 = ( 6.023 × 1023 )/(124) = 0.25 × 1023 molecules
31g P4 = 0.25 × 1023 × 31 = 1.5 × 1023 molecules .... answer
Atoms and Molecules
Q 1.
Give one major drawback of Dalton's atomic theory of matter.
Q 2.
What do we call those particles which have more or less electrons than the normal atoms ?
Q 3.
Who proposed Law of Definite Proportions (or Law of Constant Composition)?
Q 4.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 5.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 6.
(i) State the law of constant proportions.
(ii) Show that water illustrates the law of constant proportions.
Q 7.
Fill in the blanks:
Atoms can be observed using ____________ Microscope.
Q 8.
Fill in the blanks:
One mole of water contains ______________ molecules.
Q 9.
Define 'formula mass' of a compound.
Q 10.
Convert 12 g of oxygen gas into moles.
Q 11.
If 16 g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen.
Q 12.
State Law of constant proportions. Explain with an example.
Q 13.
Which postulate of Dalton’s atomic theory is the result of the law of conservation of mass?
Q 14.
Magnesium is two times heavier than C-12 atom, what shall be the mass of Mg atom in terms of atomic mass units? (Given mass of C-12 atom = 12u)
Q 15.
What is the mass of 5 moles of sodium carbonate (Na
) ?(Atomic masses : Na = 23 u ; C = 12 u ; O = 16 u)
Q 16.
Fill in the blanks:
The atomic mass of sodium is 23. The gram atomic mass of sodium is _________.
Q 17.
What is the significance of a chemical symbol?
Q 18.
Why do atoms of the most of the elements not exist independently?
Q 19.
Name one element each which forms diatomic and tetra atomic molecule.
Q 20.
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 (Na
2SO
3)?
Q 21.
Calculate the number of molecules of sulphur (S
8) present in 16 g of solid sulphur.
Q 22.
An element Z has a valency of 3. What is the formula of oxide of Z ?
Q 23.
Calculate the mass of 3.011 x 10
atoms of carbon
Q 24.
(a) What is meant by 'a mole of carbon atoms' ?
(b)(b) Which has more atoms, 50 g of aluminium or 50 g of iron ? Illustrate your answer with the help of calculations.
(Atomic masses : A1 = 27 u ; Fe = 56 u)
Q 25.
Who established the two important laws of chemical combinations?
Q 26.
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
Q 27.
Name the international organization who approves names of elements.
Q 28.
How will you define chemical symbol?
Q 29.
Fill in the blanks:
The chemical symbol of flourine is ________.
Q 30.
What are ionic compounds?
Q 31.
Work out the formula for magnesium hydrogencarbonate.
Q 32.
Name the following compounds. Also write the symbols/formulae of the ions present in them :
(a) CuSO
Q 33.
How many moles are 3.6 g of water ?
Q 34.
Calculate the number of molecules in 4 g of oxygen.
Q 35.
(a) Define gram atomic mass of a substance.How much is the gram atomic mass of oxygen ?
(b) How many moles of oxygen atoms are present in one mole of the following compounds ?
(i)Al
(ii) co
2(iii) C1
(iv) H
(p)A1
Q 36.
Fill in the blanks:
___________ is the combining capacity of an element.
Q 37.
What is relative atomic mass of an element? How it is related to atomic mass unit?
Q 38.
Based on type of substance, how molecules are classified?
Q 39.
Give three examples of monoatomic molecules.
Q 40.
Give four examples of diatomic molecules.
Q 41.
What is the molecular mass of a substance?
Q 42.
Calculate the molecular masses of the following compounds :
(a) Methane, CH
Q 43.
Calculate the molecular masses of the following compounds :
(a) Methanol, CH
Q 44.
Calculate the molecular mass of chloroform (CHC1
Q 45.
An element X has a valency of 4 whereas another element Y has a valency of 1. What will be the formula of the compound formed between X and Y ?
Q 46.
What is the mass of 0.2 mole of oxygen atoms ?
Q 47.
If one mole of nitrogen molecules weighs 28 g, calculate mass of one molecule of nitrogen in grams.
Q 48.
How many moles are there in 34.5 g of sodium ? (Atomic mass of Na = 23 u)
Q 49.
How many atoms are there in 0.25 mole of hydrogen ?
Q 50.
If 12 g of carbon is burnt in the presence of 32 g of oxygen, how much carbon dioxide will be formed?
