What volume of a 5.0 M stock solution of NH3 is needed to
make 3.0 L of 1.40 M NH3?_ _mL

840mL

Explanation:

Given parameters:

Concentration of stock  = 5M

New volume  = 3L

New concentration  = 1.40M

Unknown:

Volume of stock needed  = ?

Solution:

A concentrated standard solution which is the stock may become diluted to a lower concentration before using.

One important thing is that the number of moles of the substance in the solution before and after is the same.

  Therefore;

               C₁ V₁   = C₂ V₂

where C and V are concentration and volume

          1 and 2 are initial and final states.

      C₁  = concentration of stock

      V₁  = volume of stock

       C₂  = final concentration

      V₂ = final volume

         5 x V₁ = 1.4 x 3

               V₁ = 0.84L

This gives 840mL

the awnser is group 12