Question

Please provide formula!

Molecular weight of chemical A is 40. To make 1 liter of 1molar solution, how much should you use? Specify unit. And to make 50militer of 50millimolar solution, how much should you use?

To make 100 microliter solution that contains 10mililmolar of compund A,B,C, and D, which are in 100milimolar sotck solution individually, how much sterile water and each compound should you mix? Specify unit

A compound came in a tube containing 38.2nanomoles of dry powder. To make 100 micromolar stock solution, how much sterilie water should you add to the tube? Specify the unit.

Answer 1

Molar solution: When the amount of substance in gram equal to its molecular weight is dissolved in sufficient amount of water and then made final solution to 1000 mL (1 Ltr) it forms 1 M solution. As per this definition,

1. To make 1 L , 1 M solution of compound having molecular weight 40 the amount of compound A required will be 40 gram.

2. Now as we know that 1 L = 1000 mL and 1g = 1000 mg we can analogously say that 1 M = 1000 milimolar. Hence we can accordingly say that when the amount of substance in miligram equal to its molecular weight is dissolved in sufficient amount of water and then made final solution to 1000mL it forms 1 milimolar solution. As per this definition 40 mg i.e. 40 x 10^-3 gm = 0.04 g of A on dissolution in 1000 mL forms 1 milimolar solution

so, 50 milimolar = 40 x 50 = 2000 mg = 2 g of A ...for 1000 mL

hence = 2000 / 20 = 100 mg = 0.1 g of A .... for 50 mL (1000 mL/ 20 = 50 mL so divided by 20)

3.Now to make 100 microlitrre (100 x 10^-6 L) from 4 different solution,

Let for the required solution molarity m= 10mM and v = 100 x 10^-6L = 10^-4L

and the solutions to be used for preparing each having m₁=m2 = 3=m3 = m4 =100 mM and v1 = v2 = v3 =v4 = ?

Molarity of a solution follows, mv = m1v1 + m2v2 +m3v3+m4v4,

mv = m1v1 +m1v1+m1v1+m1v1.........(all same)

mv = 4 m1v1

10 mM x 10^-4 = 4 x 100 mLx v1

v1 = 10 x 10^-4 / 4 x 100

v1 = 2.5 x 10^-6 = 2.5 microlitre

Hence 2.5 microlitre of each A, B, C, D will be required. Totally all 4 constituent solutions makes the volume 10 microlitre and hence 90 microlitre sterile water will be required to make it 100 microlitre.

4) 38.2 nanomoles = 38.2 x 10^-3 micromoles (1 nanomoles = 10^-3 micromole) m1 = 38.2 x 10^-3 micromole , v1 = ?

m2 = 100 mM v2= 1 L

m1v1 = m2v2

38.2 x 10^-3 x v1 = 100 x 1000

v1 = 38.2 x 10^-3 / 100 x 1

v1 = 38.2 x 10^-5 L

v1 = 382 x 10^-6 L

v1 = 382 microlitre.

382 microlitre sterile water will be required.