Question

to produce 4.00 L of a 250 mM solution of sodium hydroxide, how many grams of naOH must be dissolved?

Answer 1

Multiply the volume in liters (4.00L) by the concentration of the solution in molar (0.250M) to calculate the number of moles of soluto that you need, then multiply the number of moles by the molar mass of sodium hydroxide (39.99 g/mol) to calculate the mass.

4.00L x 0.250 M x 39.99g / 1L = 39.99g

Basically you need one mol of NaOH (39.99g) dissolved in 4 liters of solution.

Answer 2

Answer: $1.56\times 10^{-3}g$

Explanation:

Molarity of a solution is defined as the number of moles of solute dissolved per Liter of the solution.

$Molarity=\frac{n\times 1000}{V_s}$

where,

Molarity = 250mM = $250\times 10^{-3}M$

n= moles of solute  

$V_s$ = volume of solution in ml = 4L = 4000 ml

${\text {moles of solute}}=\frac{\text {given mass}}{\text {molar mass}}=\frac{xg}{40g/mol}=0.025x$

Now put all the given values in the formula of molarity, we get

$250\times 10^{-3}=\frac{0.025\times x\times 1000}{4000ml}$

$250\times 10^{-3}=\frac{0.025\times x\times 1000}{4000ml}$

$x=1.56\times 10^{-3}g$

Therefore, the $1.56\times 10^{-3}g$ of NaOH must be dissolved.