Tag: percentage composition and empirical formula

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Multiple choice chemistry the language of chemistry percent composition percentage composition and empirical formula empirical formula, percentage composition and molecular formula

Calculate the number of molecules present in $0.5$ moles of magnesium oxide $\left( MgO \right) $. 

[Atomic weights :  $Mg=24, O=16$]

  1. $14.09\times { 10 }^{ 23 }$ molecules
  2. $3.0115\times { 10 }^{ 23 }$ molecules
  3. $30.12\times { 10 }^{ 23 }$ molecules
  4. $14.09\times { 10 }^{ -23 }$ molecules
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
1 mole of MgO contains Avogadro's number of molecules which is $ \displaystyle 6.023 \times 10^{23}$ molecules.

0.5 mole of MgO will contain $ \displaystyle 0.5 \times 6.023 \times 10^{23}=3.0115 \times 10^{23}$ molecules.
Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

The water of crystallisation is the fixed number of _________ molecules in one formula unit of salt.

  1. hydrogen

  2. water

  3. oxygen

  4. hydroxide
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Water of crystallisation are water molecules that form an essential part of the crystal structure of some compounds.

It is the fixed number of water molecules in one formula unit of salt.
For example Glauber's salt has 10 water of crystallisation and it's formula is $Na _2SO _4.10H _2O$.
Hence, the correct answer is option (B).

Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

Water of crystallisation is responsible for ......... of the crystals.

  1. geometric shape and colour

  2. colour

  3. geometric shape

  4. number of crystal
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Water of crystallisation are water molecules that form an essential part of the crystal structure of some compounds.

Thus, the water of crystallisation is responsible for the geometric shape and colour of the crystals.
For example, Glauber's salt (Sodium sulphate decahydrate) $Na _2SO _4.10H _2O$ has ten water of crystallisation and it is responsible for the colourless monoclinic geometry of the crystals.
Hence, the correct answer option is (A).

Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

Water of crystallization is the fixed number of water molecules chemically attached to a unit of a salt in its crystalline form.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Water of crystallization means the number of water molecules present in one formula unit of a salt.
For example: $CaSO _4.2H _2O$ is called calcium sulphate dihydrate with and it has $2$ molecules of water of crystallization.

Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

The crystalline salt, $Na _{2}SO _{4}xH _{2}O$ on heating losses $55.9\%$ of its weight. The formula of the crystalline salt is :

  1. $Na _{2}SO _{4}.5H _{2}O$
  2. $Na _{2}SO _{4}.7H _{2}O$
  3. $Na _{2}SO _{4}.10H _{2}O$
  4. $Na _{2}SO _{4}.2H _{2}O$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$Na _2SO _4.xH _2O$


Mass of hydrate $=23\times 2+32+64+18x$
$=46+96+18x$
$=142+18x$
Mass of water of hydration=$18x$
% of water in hydrate=$55.9$%

$\cfrac {18x}{142+18x}\times 100=55.9$

$1800x=7937.8+1006.2x$

$793.8x=7937.8$

$x=9.99\sim 10$

Answer= $Na _2SO _4.10H _2O$

Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

The crystalline salt ${ Na } _{ 2 }{ SO } _{ 4 }.x{ H } _{ 2 }O$ on heating loses 55.9% of its mass. The formula of crystalline salt is:

  1. ${ Na } _{ 2 }{ SO } _{ 4 }.5{ H } _{ 2 }O$
  2. ${ Na } _{ 2 }{ SO } _{ 4 }.7{ H } _{ 2 }O$
  3. ${ Na } _{ 2 }{ SO } _{ 4 }.2{ H } _{ 2 }O$
  4. ${ Na } _{ 2 }{ SO } _{ 4 }.10{ H } _{ 2 }O$
  5. ${ Na } _{ 2 }{ SO } _{ 4 }.6{ H } _{ 2 }O$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Molar mass of Na2SO4 = 142. Mass of water = 18x. Total mass = 142 + 18x. Water loss = 18x / (142 + 18x) = 0.559. Solving for x gives x = 10.

Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

State whether the given statement is true or false:

Water of crystallization is the water that occurs inside crystals.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Water of crystallization or water of hydration is water that occurs inside the crystals. It refers to water that is found in the crystalline framework of a metal or a salt that is not directly bonded to the metal cation.

Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

In $FeSO _4$.$7H _2O$, $H _2O$ represents:

  1. molar mass

  2. molecular weight

  3. molar weight

  4. water of crystallisation
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Water of crystallization or water of hydration is the water present inside the crystal. Solids are purified by the procedure called as crystallization in which after purification some pure form of water trap down inside the crystal called as water of crystallization.

Multiple choice chemistry substances in common use percent composition water of crystallisation percentage composition and empirical formula

The number of molecules of water of crystallisation present in one molecule of ferrous sulphate is_______.

  1. $5$
  2. $7$
  3. $6$
  4. $10$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

When water is used in the formation of crystals knows as water of crystallization. 'OR' 

Water of crystallization is the total weight of water in a substance at a given temperature and is mostly present in a definite ratio. 
On heating, iron(II) sulfate first loses its water of crystallization and the original green crystals are converted into a brown-colored anhydrous solid. 


$FeSO _4.7H _2O$ the heptahydrate in solution (water as solvent) transforms to both heptahydrate and tetrahydrate when the temperature reaches $56.6 ^\circ C$.


Hence, option $B$ is correct.