Tag: atoms and molecules

Questions Related to atoms and molecules

A definite mass of $ H _{2}O _{2} $ is oxidized by excess of acidified $ KMnO _{4} $ and acidified $ K _{2}Cr _{2}O _{7} $, in separate experiments. Which of the following is/are correct statements? 
(K = 39, Cr = 52, Mn = 55 )

introduction to mole gas laws and mole concept atoms and molecules chemistry mole concept chemical formula and mole concept

  1. Mass of $ K _{2}Cr _{2}O _{7} $ used up will be greater than that of $ KMnO _{4} $

  2. Moles of $ KMnO _{4} $ used up will be greater than that of $ K _{2}Cr _{2}O _{7} $

  3. Equal mass of oxygen gas is evolved in both the experiments.

  4. If equal volumes of both the solutions are used for complete reaction, then the molarities of $ KMnO _{4} $ and $ K _{2}Cr _{2}O _{7} $ solutions are in $6:5$ ratio.

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Correct Option: A,B,D
Explanation:

According to question, reaction of experiment (1) and (2)
(1)$ 5H _{2}O _2+2KMnO _{4}+3H _{2}SO _{4}\rightarrow 5O _{2}+2MnSO _{4}+K _{2}SO _{4}+8H _{2}O $


(2)$ 3H _{2}O _{2}+K _{2}Cr _{2}O _{7}+4H _2SO _{4}\rightarrow Cr _{2}(SO _{4}) _{3}+3O _{2}+K _{2}SO _{4}+7H _{2}O $

(a) According to reaction (1)
5 mole $ H _{2}O _{2} = 2\,mole KMnO _{4} $

1 mole $ H _{2}O _{2} = \dfrac{2}{5} = 0.4\,mole\,KMnO _{4} = 63.2\,g\,KMnO _{4} $

According to reaction (2)
3 mole $ H _{2}O _{2} = 1\,mole\,K _{2}Cr _{2}O _{7} $

1 mole $ H _{2}O _{2} = \dfrac{1}{3} mole\,K _{2}Cr _{2}O _{7} = 98.1 g \, K _{2}Cr _{2}O _{7} $

Mass of $ K _{2}Cr _{2}O _{7}> KMnO _{4} $

(b) Moles of $ KMnO _{4}> $ moles of $ K _{2}Cr _{2}O _{7} (0.333) $

(c)In reaction 1, 5 Mole $ H _{2}O _{2} $ released = 5 mole $ O _{2}\Rightarrow 32\times 5=160g $
In reaction 2, 3 mole $ H _{2}O _{2} $ released = 3 mole $ O _{2}=3\times 32= 96 g$ 

(d) 1 mole of $ H _{2}O _{2} = \dfrac{2}{5} $ moles $ KMnO _{4} $ exp...(1)
1 mole of $ H _{2}O _{2} = \dfrac{1}{3}$ mole $ K _{2}Cr _{2}O _{7} $ exp...(2)
$ \dfrac{KMnO _{4}}{K _{2}Cr _{2}O _{7}} = \dfrac{\dfrac{2}{5}}{\dfrac{1}{3}} = \dfrac{2}{5}\times \dfrac{3}{1} = \dfrac{6}{5}\Rightarrow 6:5 $

Options A, B and D are correct.

Two acids $ H _{2}SO _{4} $ and $ H _{3}PO _{4} $ are neutralized separately by the same amount of an alkali when sulphate and dihydrogen orthophosphate are formed, respectively. Find the ratio of the masses of $ H _{2}SO _{4} $ and $ H _{3}PO _{4} $ 

introduction to mole gas laws and mole concept atoms and molecules chemistry mole concept chemical formula and mole concept

  1. $ 1:1 $

  2. $ 1:2 $

  3. $ 2:1 $

  4. $ 2:3 $

Show answer
Correct Option: B
Explanation:

$H _{2}SO _{4}+2NaOH\rightarrow Na _{2}SO _{4}+2H _{2}O$

$H _{3}PO _{4}+NaOH\rightarrow NaH _{2}PO _{4}+H _{2}O$

Equivalent of alkali $= 19$ eq of $H _{2}SO _{4}= 1g$ eq of $H _{3}PO _{4}$

Two acids must be reacting in the ratio of their equivalent masses

Eq. wt. of $H _{2}SO _{4}=\dfrac{98}{2}=49$

Eq. wt. of $H _{3}PO _{4}= \dfrac{98}{1}=98$

$\therefore $ ratio of masses of $H _{2}SO _{4}$ & $H _{3}PO _{4}$
$49:98=1:2$

$\Rightarrow 1:2$

A gaseous alkane is exploded with oxygen. The volume of ${O} _{2}$ for complete combustion of alkane to $C{O} _{2}$ formed is in the ratio $7:4$. The molecular formula of alkane is:

introduction to mole gas laws and mole concept atoms and molecules chemistry mole concept chemical formula and mole concept

  1. ${C} _{2}{H} _{6}$

  2. ${C} _{3}{H} _{8}$

  3. ${C} _{4}{H} _{10}$

  4. $C{H} _{4}$

Show answer
Correct Option: A
Explanation:

The balanced reaction is given below:


${C} _{n}{H} _{2n+2} +[n+\displaystyle\frac{n+1}{2}]{O} _{2}\rightarrow nC{O} _{2} +(n+1){H} _{2}O$

Given, 

$\displaystyle\dfrac{n+\dfrac{n+1}{2}}{n}=\dfrac{7}{4}\implies n=2$

Hence, the alkane is ${C} _{2}{H} _{6}$.

Hence, the correct option is $A$

Let 'p' be the initial pressure of a gas. If the volume of a given mass of the gas, at constant temperature, becomes three times, the pressure will be:

chemistry atoms and molecules avogadro hypothesis avogadro's law avogadro law

  1. 3p

  2. p/3

  3. 9p

  4. p

Show answer
Correct Option: B
Explanation:

According to Boyle's law,

$P _1V _1=P _2V _2$
$pV _1=P _2.3V _1$
$\Rightarrow P _2=p/3$

At constant temperature, in a given mass of an ideal gas:

chemistry atoms and molecules avogadro hypothesis avogadro's law avogadro law

  1. the ratio of pressure and volume always remains constant

  2. volume always remains constant

  3. pressure always remains constant

  4. the product of pressure and volume always remains constant

Show answer
Correct Option: D
Explanation:

Boyle's Law states that, at constant temperature, the product of the pressure and volume of a given mass of an ideal gas in a closed system, is always constant.

A mixture of helium and argon contains 3 moles of $He$ for every 2 moles of $Ar$. The partial pressure of argon in the mixture is

chemistry atoms and molecules avogadro hypothesis avogadro's law avogadro law

  1. two thirds the total pressure

  2. one third the total pressure

  3. three fifths the total pressure

  4. two fifths the total pressure

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Correct Option: D
Explanation:

The partial pressure of any gas is the product of mole fraction and total pressure


$P _{Ar}=\dfrac{2}{2+3}.P _{total}$

The volume of 2.8 g of $CO$ at $27^0C$ and 0.821 atm pressure is:

chemistry atoms and molecules avogadro hypothesis avogadro's law avogadro law

  1. 1.5 litre

  2. 3 litre

  3. 30 litre

  4. 0.3 litre

Show answer
Correct Option: B
Explanation:

Molecular mass of $CO$ = 28. 

Moles of $CO$ present $= \dfrac{2.8}{28}$
 
$V=\dfrac{nRT}{P}=\dfrac{\dfrac{2.8}{28}\times0.0821\times300}{0.821}=3\ litres$

At what temperature will both Celsius and Fahrenheit scales read the same value:

chemistry atoms and molecules avogadro hypothesis avogadro's law avogadro law

  1. 100

  2. 180

  3. 40

  4. -40

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Correct Option: D
Explanation:

We know, $\dfrac{F-180}{212-180}=\dfrac{C-273}{373-273}$


$F=C=-40$

So, the correct option is $D$

An isobaric process is associated with:

chemistry atoms and molecules avogadro hypothesis avogadro's law avogadro law

  1. constant temperature

  2. constant volume

  3. constant pressure

  4. small change in enthalpy

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Correct Option: C
Explanation:

Isobaric process refers to a process taking place at constant pressure.

In the ideal gas equation, the unit of universal gas constant R is?

chemistry atoms and molecules avogadro hypothesis avogadro's law avogadro law

  1. Mole-atm/K

  2. Litre/mole

  3. Litre-atm/mole.K

  4. erg/K

Show answer
Correct Option: C
Explanation:

In ideal gas equation,


$R=\dfrac{PV}{nT}$ where, 
$P$ is the pressure in 'Atmosphere (atm)'
$V$ is the volume in Litre
$n$ is the number of moles
$T$ is the Temperature in Kelvin (K) 

$ \therefore $Units of $R$= litre.atm/mol.K