Some chemistry help please?

Question

Okay, I don't understand these things:

If you have o.500 mol of KCL, some distilled water, and a 250. mL volumetric flask, how can you make a 0.500 M solution of KCl?

I think it's the concentration formula, Cmolarity= mol of solute / volume of solution, but you would need a 1 Liter flask wouldn't you??

and which of these has a greater mass of solute, 1 liter of 0.1 M NaCl, or 1 L of 0.06 M of Na2CO3? :| please help?

thank you

Answer 1

Volume = 250 mL = 0.250 L
M = moles / V
moles = M x V = 0.500 x 0.250 = 0.125
Molar mass KCl = 74.553 g/mol
Grams = mol x molar mass = 0.125 mol x 74.553 g/mol =9.32 g
We put 9.32 g of KCl in the volumetric flask , then we add enought distilled water to reach 250 mL

1 L of 0.1 M NaCl => 0.1 mol of NaCl
Molar mass = 58.44 g/mol
0.1 mol of NaCl => 5.844 g

1 L of 0.06 M of Na2CO3 => 0.06 mol of Na2CO3
Molar mass = 105.99 g/mol
0.06 mol Na2CO3 => 0.06 mol x 105.99 g/mol = 6.36 g

1 L of Na2CO3 0.06 M has a greater mass of solute

Answer 2

Ya gotta think outside the liter flask. M means moles per liter. It doesn't take a rocket scientist to realize you can scale 0.5M to 0.05 moles per 100 cc or, hmmmm, 0.125 moles per 250 cc.

Figure out the mole weights of each. Then multiply each mole wt by the Molar value to get your answers in grams.