Question

What mass of NaF must be mixed with 25ml of water to create a 3.5% by mass solution

Answer 1

Answer:

0.91 g NaF

Explanation:

Mass percentage is one way of representing the concentration of an element in a compound or a component in a mixture. Mass percentage is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100%.

Let mass of NaF = x g

∴ mass percent = mass of NaF / mass of total mixture *100

3.5 = (x/ x + 25) * 100

∴ x = 0.91 g

Answer 2

Answer: The mass of NaF that must be mixed with 25 mL of water is 0.91 grams

Explanation:

We are given:

Volume of water = 25 mL

To calculate the mass of water, we use equation used to calculate density:

$\text{Density of substance}=\frac{\text{Mass of substance}}{\text{Volume of substance}}$

Density of water = 1 g/mL

Volume of water = 25 mL

Putting values in above equation, we get:

$1g/mL=\frac{\text{Mass of water}}{25mL}\\\\\text{Mass of water}=(1g/mL\times 25mL)=25g$

We are given:

3.5 % (m/m) NaF solution

This means that 3.5 grams of NaF is present in 100 g of solution

Mass of water in solution = 100 - 3.5 = 96.5 g

To calculate the mass of NaF mixed with 25 mL of water, we use unitary method:

In 96.5 grams of water, the mass of NaF added is 3.5 grams

So, in 25 grams of water, the mass of NaF that must be added will be = $\frac{3.5}{96.5}\times 25=0.91g$

Hence, the mass of NaF that must be mixed with 25 mL of water is 0.91 grams