How many grams of sodium hydroxide (NaOH) are needed to prepare 250 mL of a 0.20 M (mol/dm3) sodium hydroxide solution?

How many mols do you want? That's M x L = ?

Then mols = grams/molar mass. You know molar mass and mols, solve for grams.

To calculate the number of grams of sodium hydroxide (NaOH) needed to prepare a solution, we need to use the formula:

Number of moles = concentration (mol/dm^3) x volume (dm^3)

First, let's convert the volume from mL to dm^3:

250 mL = 250/1000 = 0.25 dm^3

Now we can substitute the given values into the formula:

Number of moles = 0.20 mol/dm^3 x 0.25 dm^3
Number of moles = 0.05 mol

To convert from moles to grams, we need to know the molar mass of sodium hydroxide, which consists of one atom of sodium (Na), one atom of oxygen (O), and one atom of hydrogen (H).

Molar mass of NaOH = 22.99 g/mol (Na) + 16.00 g/mol (O) + 1.01 g/mol (H)
Molar mass of NaOH = 40.00 g/mol

Now we can calculate the number of grams:

Number of grams = Number of moles x Molar mass
Number of grams = 0.05 mol x 40.00 g/mol
Number of grams = 2.00 grams

Therefore, you would need 2.00 grams of sodium hydroxide to prepare 250 mL of a 0.20 M sodium hydroxide solution.