Question

How many grams of NACL molecular weight (58.44) is needed to make 2.5 L of a 0.4 M solution ?

Answer 1

For this problem we need to do the conversion of volume and concentration to moles of solute and the to gr. First, we determine the number of moles of NaCl that we have in 2500 ml of solution at 0.4 M

moles of NaCl = 2500ml * 0.4 mol NaCl / 1000ml = 1 mol NaCl

The molar mass of NaCl is 58.44, so that, the necessary mass is calculate

gr NaCl = 1 mol NaCl * 58.44 g NaCl / 1 mol NaCl

gr NaCl = 58.44 gr
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This is another approach to check this answer above:

Molarity is defined as:

M = solute moles/solution liters.

M = (wt/m.wt * 1000 ml) / v

wt= weight of substance
m.wt = molecular weight
v = volume

Clearing the incognito;

0.4 = (wt/58.44 *1000 ml)/(2500 ml)
w = (0.4 * 2500 * 58.44) / 1000
w = 58.44 gr NaCl

Regards!