Final answer:
To convert from mg to ml, knowing the density or concentration of the substance is necessary.
Step-by-step explanation:
To convert from milligrams (mg) to milliliters (ml), we need to know the density or concentration of the substance. In this case, we are given that the nurse is preparing to administer 40mg of a substance in 0.5oz (which is approximately 14.8ml). However, without the density or concentration of the substance, we cannot accurately convert the amount from mg to ml.