Given below are the Class 9 Science CBSE worksheet for Atoms and Molecules
(a) Short questions
(b) True and False problems
(c) Fill in the blank's
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Fill In The Blanks
- During a chemical reaction, the sum of the _________ of the reactants and products remain unchanged.
- In a pure chemical compound, elements are always present in a ________ proportion by mass.
- Clusters of atoms that act as an ion are called __________ ions.
- In ionic compounds, the charge on each ion is used to determine the ________of tye compound.
- The Avogadro constant _________ is defined as the umber of atoms in exactly ________ of crbon-12
- Mass of 1 mole of a substance is called its __________.
- The abbreviation used for length names of elements are termed as their _______.
- A chemical formula is also known as a ________.
- Those ions which are formed from single atoms are called _________.
- Ionic compounds are formed by the combination between ________ an ______.
- The valency of an ion is _________ to the charge on the ion.
- Mole is a link between the __________ and __________.
- The SI unit of amount of a substance is __________.
ANSWERS
- Masses
- Definite
- Polyatomic
- Chemical formula
- 86.022 x 1023, 12g
- Molar mass
- Symbol
- Molecular formula
- Simple ins
- Metal and non- metals
- Equal
- Mass of atoms & number of atoms
- Mole
True / False
- Formula mass of $Na_2O$ is 62 amu.
- Those particles which have more or less electrons than the normal atoms are called ions.
- Formula for sulphur dioxide is $SO_3$.
- Molar mass of ethyne ($C_2H_2$) is 26 g/mol.
- 22 gm of $CO_2$ consists of 1 mole.
- Number of molecules in 32 gram of oxygen is $6.02 \times 10^{23}$.
- Water is an atom.
- Formula for sulphur dioxide is $SO_2$.
- Clusters of atoms that act as an ion is called polyatomic ion.
- Mass of 1 mole of a substance is called its formula mass.
- In a pure chemical compound, elements are always present in a definite proportion by mass.
ANSWERS
- True
- True
- False
- True
- False
- True
- False
- True
- True
- False
- True
Very Short Answer Questions
1. Define term mole.
Solution
The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.
2.What is Avogadro's Number?
Solution
The Avogadro constant $6.022 \times 10^{23}$ is defined as the number of atoms in exactly 12 g of carbon-12.
3.What is gram atomic weight?
Solution
Gram atomic weight is the mass in gram of 1 mole of atoms in a mono-atomic chemical element
4. What is the difference between $CO_2$ and $2CO_2$?
Solution
$CO_2$ means 1 molecule of carbon dioxide $2CO_2$ means 2 molecules of carbon dioxide
5. Name the following compounds $PC_3$ and $SO_2$.
Solution
Phosphate Carbon and Sulphur dioxide
6. In the smallest whole-number ratio must N and O atoms combine to make dinitrogen to tetroxide $N_2O_4$? What is the mole ratio of the elements in this compound?
Solution
1:2
7.How many moles of sodium atoms correspond to $1.56 \times 10^{21}$ atoms of sodium?
Solution
Sodium atoms = $1.56 \times 10^{21}$
Avogadro number = $6.022 \times 10^{23}$
$Moles = \frac {1.56 \times 10^{21}} { 6.022 \times 10^{23}}$
$Moles = 0.26 \times 10^{-2}$
8. What is the relationship between the formula weight of a substance and its molar mass?
Solution
The molar mass of any substance is its atomic mass, molecular mass, or formula mass in grams per mole.
9. How many grams of silver are in 0.263 mol of g?
Solution
1 Mole of Silver is 107.86 gm
0.263 mol of silver will be = 107.86 * 0.263=28.367 gm
10.How many atoms are $1.00 \times 10^{-9}$ g of lead?
Solution
1 mol of Pb = 207.2g
or 207.2 g has $6.022 \times 10^{23}$ atoms
$1.00 \times 10^{-9}$ will have = $\frac {6.022 \times 10^{23}}{207.2 g} \times {1.00 \times 10^{-9}}=2.91 \times 10^{12}$ atoms.
11.How many grams of iron are needed to combine with 25.6g of O to make $Fe_2O_3$?
Solution
Molecular weight of Fe = 55.85
Molecular weight of O = 16.00
Molecular weight of $Fe_2O_3 = (55.85) \times 2 + (16) \times 3 = 111.7+48 = 149.70$
For 48 gm O, Fe required is 111.7 gm
So for 25.6 gm O, Fe required is 59.57 gm
12.What is the mass of 4 moles of aluminium atoms? ( Atomic mass of AI = 27u)
Solution
Mass of 1 mole of aluminium atoms = 27 g. Mass of 4 moles of aluminium atoms = (27 x 4) = 108 g.
13.Calculate the mass of $6.022 \times 10^{22}$ atoms of He.
Solution
$6.022 \times 10^{23}$ atom of Helium weight 4g
Therefore $6.022 \times 10^{22}$ atoms of Helium will weighs
$= \frac {4 \times 6.022 \times 10^{22}}{6.022 \times 10^{23}} =0.4g$
14.Calculate the number of moles in $3.011 \times 10^{22}$ molecules of carbon dioxide.
Solution
$6.022 \times 10^{23}$ is 1 mole
$3.011 \times 10^{22}$ will be .05 mole
15.A sample of 45.8g of $H_2SO_4$ contains how many moles of $H_2SO_4$?
Solution
molar mass of $H_2SO_4$ = 98.078 g/mol
No of moles = $\frac {45.8}{98.078}=0.467$ moles
16.What s the mass in grams of 5 moles of Fe?
Solution
Molecular mass of Fe= 55.845 g/mol
1 mole of Fe weighs= 55.845g
5 moles of Fe weigh=$55.845 \times 5 =279.225$ g
17.How many moles of NaCI are present in 20 gm of the substance?
Solution
Molar mass of $NaCl$ = 58.4 g/mol
Number of moles in 20.0 g NaCl is = $\frac {20}{58.4}= .342 $ mol
18.How many grams of O is present in 50 gm of $CaCO_3$?
Solution
Molecular mass of $CaCO_3$ = $40 + 12 + 16 \times 3=100$
No of moles in 50 gram = $\frac {50 }{ 100} =0.5$
1 $CaCO_3$ molecule has three oxygen atoms so one mole $CaCO_3$ atom has 3 mole oxygen atom or 48 gm of oxygen
So .5 moles will have = .5 *48 =24 grams of oxygen
19.How many grams of $CO_2$ are present in 0.1 mole $CO_2$?
Solution
molecular mass of $CO_2$ = 44
Mass = number of moles * molecular mass
= 0.1 * 44= 4.4 g
20.Describe the difference between the mass of a mole of oxygen atoms (O) and the mass of a mole of oxygen molecules ($O_2$).
Solution
Mass of mole of Oxygen atom , i.e one O is present = O = 16 grams
Mass of mole of $O_2$ ,Oxygen gas = $O_2$= 16 + 16 = 32 grams
So a mole of oxygen atoms has only half the mass of a mole of oxygen molecules
21.What do you understand by the term Formula unit?
Solution
A formula unit in chemistry is the empirical formula of any ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. It is the lowest whole number ratio of ions represented in an ionic compound.
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