Verified answer

Look up atomic mass of C, H and O ... multiply by subscripts and add... this gives the molar mass.

12 X 12 + 22 X 1 + 11 X 16

= 144 + 22 + 176

= 342 .... 1 mole = 342 g <<< molar mass

500mL X (1L/1000mL) X (0.3 mole/L) X (342g/mole)

... use a calculator .. answer is grams of lactose

weight grams of lactose from above and dissolve that into enough water (volumetric flask) to make a total volume of 500 mL