Question #9c132

1 Answer
Apr 23, 2016

Answer:

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