Question

11. How many grams of talc should be added to 1 lb of a powder containing 20 g of zinc undecylenate per 100 g to reduce the concentration of zinc undecylenate to 3% w/w? 3g

Answer 1

There are roughly 454 grams in pound. So, there are 454 grams of powder.

Now, the power contains 20 grams of zinc undecylenate per 100 grams, so the total amount of zinc undecylenate is given by

$\small {20\over 100}\times 454 = 90.8$

Thus, the total amount of zinc undecylenate in the power is 90.8 grams. Now, for the concentration of zinc undecylenate to be 3% w/w, 90.8 grams has to be 3% of the total weight of the mixture, so the total weight is given by

$\small {90.8\over \text{total weight}}\times 100 =3 \Rightarrow \text{total weight}={9080\over 3}=3026.6667$

Thus, for the concentration of zinc undecylenate to be 3% w/w, the total weight of the power must be 3026.6667 grams, but the weight of the power is 454 grams. Thus, we need to add

$\small 3026.6667-454=2572.67$

roughly 2572.67 grams of talc