Question

Calculate the wt.in gram & no.of molecules in 0.5 moles of Ca(oh)2

Answer 1

The weight in grams is 37 g

and the number of molecules is 3.0115 × 10^23.

Given: The number of moles of Ca(OH)2 = 0.5

To Find: The weight in grams & number of molecules

Solution:

Molecular mass of Ca(OH)2 = 40 + 2 × ( 16 + 1 )

                                              = 74 g

We know that,

1 mole of Ca(OH)2 = 74 g

So, 0.5 moles of Ca(OH)2  = ( 74 × 0.5 ) g

                                            = 37 g

Again, we know that

1 mole of Ca(OH)2 contains = 6.023 × 10^23 molecules

So, 0.5 moles of Ca(OH)2  = ( 6.023 × 10^23 × 0.5 ) molecules

                                            = 3.0115 × 10^23 molecules

Hence, the weight in grams is 37 g

and the number of molecules is 3.0115 × 10^23.

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Answer 2

The weight in grams for 0.5 moles of $Ca(OH)_2$  is 37 g and the number of molecules in the same 0.5 moles is $3.0115\times10^{23}$ molecules.

• The mole concept is a concept in chemistry which was introduced to provide a method of expressing substances as an amount of atoms present in it.
• At the molecular level, even if a small fraction of an element or a sample is taken it can be observed that, that portion of the element also consists of several number of atoms.
• Therefore, in order to measure this amount of atoms or entities present in some sample was termed as one 'mole'.
• A mole is defined to be the equivalent of the amount of substance that consists of the same amount of units or entities contained in exactly 12 g of a Carbon atom.
• The amount of atoms contained in 12 g of a Carbon atom is given exactly by a constant known as the Avogadro number or the Avogadro constant whose standard value is fixed given to be . The Avogadro number is denoted as .
• So the mole of every element is compared with the mass and the number of atoms present in a carbon-12 sample.
• Here, the number of moles of the given sample is 0.5 moles.
• The equivalent weight in grams as well as the number of molecules contained in these 0.5 moles of  $Ca(OH)_2$.
• To find the gram equivalent we first need to find the molecular mass of the given compound.
• Molecular mass of a compound is calculated by summing the individual atomic masses w.r.t the number of the elements present in the compound.

We know that,

Atomic mass of Ca: 40

Atomic mass of H: 1

Atomic mass of O: 16

Since there are 2 molecules of OH it is multiplied with 2.Hence, the molecular mass of $Ca(OH)_2$ will be given as:

Thus, molecular mass of $Ca(OH)_2= 40 + 2 \times ( 16 + 1 )$

Molecular mass = $40+2(17)=40+34$

∴ Molecular mass of $Ca(OH)_2$ = 74 g.

Thus, the sample of $Ca(OH)_2$ will contain one mole as this is equivalent to the amount of sample considered that is 74 g.

So, 1 mole of $Ca(OH)_2$  = 74 g

⇒0.5 moles of $Ca(OH)_2$  = ? g

The weight in grams for 0.5 moles of $Ca(OH)_2$  is to be found out and hence we have:

$=0.5\times74=37 g$ equivalent.

Thus, weight in grams for 0.5 moles of $Ca(OH)_2$  is 37 g.

Now, we need to find the number of molecules in 0.5 moles of $Ca(OH)_2$ . We know from the above mole concept that:

1 mole = $6.022\times 10^{23}$ molecules

⇒0.5 moles = ? molecules

The number of molecules in 0.5 moles of the sample will be:

$=0.5\times6.022\times 10^{23}$

$=3.0115\times10^{23}$ molecules.

Thus, number of molecules in 0.5 moles of $Ca(OH)_2$  is $3.0115\times10^{23}$ molecules.

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