Question

1. How many milliliters of a 1:20 w/v solution of copper sulfate should be used to prepare 1 L of a 0.50% w/v solution?

2. How many grams of lidocaine should be added to a 2 % lidocaine ointment to prepare 1000 g of 5% of lidocaine ointment? (Mixing pure lidocaine and 2% lidocaine ointment)

Answer 1

1. How many milliliters of a 1:20 w/v solution of copper sulfate should be used to prepare 1 L of a 0.50% w/v solution?

We need to apply dilution law, which is based on the mass conservation principle

initial mass = final mass

this apply for moles as weel ( if there is no reaction, which is the case )

mol of A initially = mol of A finally

or, for this case

moles of A in stock = moles of A in diluted solution

Recall that

mol of A = Molarity of A * Volume of A

then

moles of A in stock = moles of A in diluted solution

Molarity of A in stock * Volume of A in stock = Molarity of A in diluted solution* Volume of A in diluted solution

Now, substitute known data

C1 = 1.2 % m/v

V1 = ?

C2 = 0.5 %

V2 = 1 L

C1*V1 = C2*V2

1.2*V1 = 0.5*1

V1 = 0.5/1.2 = 0.41666 Liters = 416.7 mL

2. How many grams of lidocaine should be added to a 2 % lidocaine ointment to prepare 1000 g of 5% of lidocaine ointment? (Mixing pure lidocaine and 2% lidocaine ointment)

(5-2) = 3% required

if mass = 1000 g

then-- >3/100*1000 = 30 g

we need to add 30 grams to form a 5% lidocaine mix at 1000 g