Atoms and Molecules Test

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Atoms and Molecules Test - 54

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Weekly Quiz Competition

Question 1
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Which of the following is a quantity necessary to determine the molecular formula of a compound?

A
Boiling point

B
Rate of reaction

C
Molecular mass

D
Molarity (moles per litre)

E
Density

Solution

Molecular mass is a quantity necessary to determine the molecular formula of a compound.
From elemental analysis, we can determine the percentage composition of various elements present in the sample. From percent composition, we can calculate the empirical formula. But to convert empirical formula into molecular formula, we need molecular mass. The ratio of the molecular mass to the empirical formula mass gives the number of empirical formula units present in one molecular formula unit.
Question 2
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What is the molar mass of ammonium carbonate $${({NH}_{4})}_{2}{CO}_{3}$$?

A
$$48\ g/mol$$

B
$$96\ g/mol$$

C
$$82\ g/mol$$

D
$$78\ g/mol$$

E
$$192\ g/mol$$

Solution

$$(NH_4)_2CO_3$$ is the chemical formula of ammonium carbonate.

$$N = 14 \times (2) = 28$$
$$H = 1 \times (4 \times 2) = 8 $$
$$C = 12 \times 1 = 12 $$
$$O = 16 \times 3 = 48 $$

Molar mass $$= 28 + 8 + 12 + 48 = 96\ g/mol$$
Question 3
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How many grams are there in a $$7.0$$ mole sample of sodium hydroxide?

A
$$40.0g$$

B
$$140.0\ g$$

C
$$280.0\ g$$

D
$$340.0\ g$$

E
$$420.0\ g$$

Solution

The formula of sodium hydroxide is $$NaOH$$
Atomic weight of $$Na$$ = $$23\ {g}/{mol}$$
Atomic weight of $$ O $$ = $$16\ {g}/{mol}$$
Atomic weight of $$H$$ = $$ 1\ {g}/{mol}$$
$$\therefore $$ molecular weight of $$NaOH$$ = atomic mass of $$Na$$ + atomic mass of $$O$$ + atomic mass of $$H$$
= $$23 + 16 + 1 = 40\ {g}/{mol}$$
Therefore, 1 mole of $$NaOH = 40\ g/mol$$
7 moles of $$ NaOH = 7 \ mole \times 40\ g/mol= 280g$$

Hence, option $$B$$ is correct.
Question 4
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Which of the following contains the least number of molecules?

A
52 g of $$N_2O_5$$

B
36 g of $$H_2O$$

C
28 g of $$CO$$

D
51 g $$NH_3$$

Solution

Let N be the Avogadro's number.
Mass is divided with molar mass to obtain the number of moles.
The number of moles are multiplied with Avogadro's number to obtain number of molecules.

(A) Number of molecules in 52 g of $$\displaystyle N_2O_5 = \dfrac {52 \: g}{(28+80) \: g/mol} \times N = 0.48N$$

(B) Number of molecules in 36 g of $$\displaystyle H_2O = \dfrac {36 \: g}{18 \: g/mol} \times N = 2N$$

(C) Number of molecules in 28 g of CO $$\displaystyle = \dfrac {28 \: g}{28 \: g/mol} \times N = N$$

(D) Number of molecules in 51 g $$\displaystyle NH_3 \: = \dfrac {51 \: g}{17 \: g/mol} \times N = 3N$$

Hence, 52 g of $$\displaystyle N_2O_5$$ contains least number of molecules.
Question 5
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The reaction between dinitrogen monoxide, $$N_{2}O$$ and oxygen gas, $$O_{2}$$, carried out at high pressures, results in the formation of dinitrogen tetroxide, $$N_{2}O_{4}$$, as the only product. The unbalanced equation for the reaction is:

$$N_{2}O(g) +O_{2}(g)\rightarrow N_{2}O_{4}(g)$$

Given the legend and the $$N_{2}O/O_{2}$$ mixture shown below.
Which of the following represents the composition of the system after the reaction has gone to completion?

A

B

C

D

Solution

Balanced reaction:

$$N_{2}O(g) +1.5O_{2}(g)\rightarrow N_{2}O_{4}(g)$$

1 molecule of $$N_2O$$ reacts with 1.5 molecules of oxygen as per the balanced reaction.

6 molecules of $$N_2O$$ reacts with 9 molecules of oxygen as per the balanced reaction.

Since the reactant is present in stoichiometric ratio so all the reactants will be converted to product i.e. 6 $$N_2O_4$$ molecules.
Question 6
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A student carries out the following reaction in the laboratory:

Sodium carbonate + Copper sulphate $$\rightarrow $$ Copper carbonate + Sodium sulphate

How could you determine the reaction obeyed the law of conservation of mass?

A
Mass of the sample after the reaction is compared to the mass before the reaction begins

B
Mass of the sample as the reaction is run is measured

C
Filtering the reactants

D
Running the reaction in an open system

Solution

Law of conservation of mass states that mass of the reactants before the reaction should be same as the mass of the products after the reaction.

We will measure the mass of the product and see if it is the same as the mass of the reactants. Both will be same, thus showing the law of conservation of mass is obeyed.

$$\Rightarrow$$ Mass of the reactants = Mass of the products after the reaction
Question 7
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Identify the anion from $$KClO_3$$.

A
$$Cl^-$$

B
$$O^{2-}$$

C
$$ClO_3{^-}$$

D
None of the above

Solution

The cation is $$ \displaystyle K^+$$ ion and the anion is $$ \displaystyle ClO_3{^-}$$ ion.

$$ \displaystyle KClO_3 \rightarrow K^+ +ClO_3{^-}$$
Question 8
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The total number of atoms present in $$CO_3^{2-}$$ is__________.

A
5

B
3

C
4

D
2

Solution

The total number of atoms present in carbonate ion $$CO_3^{2-}$$ is four. It includes one $$C$$ atom and three $$O$$ atoms. Carbonate ion is a polyatomic ion.
Question 9
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Which of the following would weight the least?

A
$$2$$ gram atoms of nitrogen

B
$$1$$ mole of silver

C
$$22.4$$ liters of oxygen gas at $$1$$ atmospheric pressure and $$273$$ K.

D
$$6.02 \times 10^{23}$$ atoms of carbon.

Solution

A] 2g atoms of nitrogen means $$2\times 14 =$$ 28g

B] 1 mole of silver = 107.8g.

C] 22.4 litres of oxygen = 32g.

D] $$6.02\times 10^{23}$$ atoms = 1 mole = 12 g of carbon.

So, option $$D$$ has the least weight.
Question 10
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Compound is different from molecule because, _________.

A
compound is the substance which is made up of molecules

B
molecule part of a compound

C
molecule is a measurable quantity

D
All of these

Solution

Molecule is the smallest part of the compound which contains all properties of compound. So, compounds are made up of molecules. We can measure the number of molecules in a given mass of a compound.

Water is a compound, which is made of water molecules having two hydrogen and one oxygen atom $$(H_2O)$$

Hydrochloric acid is a compound whose molecules are made of one atom of hydrogen and one atom of chlorine $$(HCl)$$.