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Question 1 of 3

A toothpaste contains 0.8 % w/w Triclosan. If a person puts 0.5 g of the toothpaste on the toothbrush, how many milligrams of Triclosan are there on the toothbrush?

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mg
There are 4 mg of Triclosan on the toothbrush.
EITHER
 mass of component = % w/w x mass of product    g
100
mass of Triclosan = 0.8 x    0.5      g
100
mass of Triclosan = 0.004 g = 4 mg
OR
 0.8 % w/w Triclosan means that:
100 g of toothpaste contains 0.8 g of Triclosan
Thus: 0.5 g of toothpaste contains    0.5    x 0.8 g of Triclosan
100
0.5 g of toothpaste contains 0.004 g ie 4 mg of Triclosan

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Question 2 of 3

How many grams of Salicylic Acid would you need to weigh if you had to prepare 600 g of Salicylic Acid Ointment BP 1988 containing Salicylic Acid at 2 % w/w?

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g
You would need to weigh 12 g of Salicylic Acid.

EITHER
 mass of component = % w/w x mass of product    g
100
mass of Salicylic Acid = 2 x 600   g
100
mass of Salicylic Acid = 12 g
OR
 2 % w/w Salicylic Acid means that:
100 g of ointment contains 2 g of Salicylic Acid
Thus: 600 g of ointment contains   600  x 2 g of Salicylic Acid
100
600 g of ointment contains 12 g of Salicylic Acid

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Question 3 of 3

Calamine Ointment BP 1988 contains 15 % w/w Calamine and the rest is made of a hydrophobic basis. If you need to prepare 660 g of the ointment, what is the amount of hydrophobic basis you need?

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grams
You would need 561 grams of hydrophobic basis.
You can approach this problem in 2 ways:
1 Work out the mass of Calamine needed. The amount of hydrophobic basis needed is then 660g less the mass of Calamine. Mass of Calamine needed:
15 % w/w means that:
100 g of ointment contains 15 of Calamine
660 g of ointment contains 660 x 15 = 99 g of Calamine
100
Therefore mass of hydrophobic basis = 660 - 99 = 561 grams
2 Because you know that the ONLY other component is the hydrophobic basis, you
can calculate its % w/w and calculate the mass from this. % w/w = 100 - 15 = 85
85 % w/w hydrophobic basis means that:
100 g of ointment contains 85 g of basis
660 g of ointment contains 660 x 85 = 561 grams of basis
100

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