Question

Question #9c132

Answer 1

We just may apply expression for molarity.

Explanation:

According to molarity (or molar concentration) expression:

`M = n/V = "number of solute moles" / "litres of solution"`

We know that `V = 200 " ml"`, now we need the number of moles for a 0.2 M solution:

`0.2 " M" = {n ("C"_16 "H"_18 "N"_3 "SCl")}/{0.2 " l"} rightarrow`

`rightarrow n ("C"_16 "H"_18 "N"_3 "SCl") = 0.04 " mol"`

Now, let us calculate the mass of `"C"_16 "H"_18 "N"_3 "SCl"` that we need to reach 0.04 mol. For this, we just take in account the molar mass of every elements:

• For carbon, C, we know that there are 16 atoms for every molecule of methylene blue. Its atomic mass is 12.01 amu (atomic mass units), so there are `12.01 cdot 16 = color(blue) 192.16` amu of C per molecule. Analogously, we can say that there are 192.16 g of C per mol of methylene blue (atomic mass on amu and molar mass on g/mol are equivalent).

We just may apply this method to all the elements on methylene blue molecule:

• For hydrogen: `1.01 cdot 18 = color(blue) 18.18` g/mol.
• For nitrogen: `14.01 cdot 3 = color(blue) 42.03` g/mol.
• For sulphur: `32.07 cdot 1 = color(blue) 32.07` g/mol.
• For chlorine: `35.43 cdot 1 = color(blue) 35.43` g/mol.

The sum of all being:

`192.16+18.18+42.03+32.07+35.43 = color(green) 319.87` g/mol of molecule of methylene blue

If we need 0.04 mol of `"C"_16 "H"_18 "N"_3 "SCl"`, and each mole has a mass of 319.87 g, we will need:

`0.04 cancel " mol" cdot {319.87 " g"}/{1 cancel " mol"} = color(red) 12.7948 " g"` of methylene blue