biology question

Question

To extract your DNA from your cheek cells, we prepared a solution of 0.9% NaCl (wt/vol) for you. If you start with salt crystals, how you would make 500 mL of 0.9% NaCl?

A. Dissolve 9 g NaCl in water and obtain a final volume of 500 mL 

B. Dissolve 4.5 g NaCl in water and obtain a final volume of 500 mL 

C. Dissolve 0.9 g NaCl in water and obtain a final volume of 500 mL 

D. Dissolve 0.45 g NaCl in water and obtain a final volume of 500 mL 

Thank you.

Answer 1

Hi Jas A.!

A weight/volume (w/v%) percentage, or mass/volume percentage (m/v%) concentration is one measure among many for concentration. To calculate the concentration, you simply divide the mass of the solute in grams by the volume of solution in milliliters, and then multiply by 100%:

(w/v%) = mass of solute (g)/volume of solution (mL) x 100%

In our case, we know the w/v % and we know the volume of solution. However, we must find the mass in grams of the solute, NaCl.

(w/v%) = mass of NaCl/volume of solution x 100% => mass of NaCl = [(w/v%)*volume]/100%

<=> (0.9%*500mL)/100%

mass of NaCl = 4.5g

Hope this helps!

Cheers