First figure out how much substance is in 500 microliters of a 2 mg/L solution this is 0.0005 L X 2mg/L =0.001 mg Then figure out how much solution you need to get 0.001 mg which is 0.001 mg divided by 1 mg/mL which equals 0.001 mL or 1 microlitre.. So to make your solution you would add 1 microliter of your stock to 499 of water.