Question

A hydrate is compound that included water molecules within its crystal structure. During an experiment to determine the percent by mass of water in a hydrate crystal, a student found the mass of the hydrated crystal to be 6.235g. After heating to constant mass, the mass was 4.90g. What is the percent by mass of water in this crystal? *

22%
21.4%
78.5%
27%

Answer 1

The percent by mass of water in this crystal is:

• 21.4%

Step-by-step explanation:

This exercise can be easily solved using a simple rule of three where the initial weight of the hydrated crystal (6,235 g) is taken into account as 100% of the mass, and the percentage to which the mass of 4.90 g corresponds (after getting warm). First, the values and unknown variable are established:

• 6,235 g = 100%
• 4.90 g = X

And the value of the variable X is found:

• X = (4.90 g * 100%) / 6,235 g
• X = approximately 78.6%.

The calculated value is not yet the percentage of the water, since the water after heating the glass has evaporated, therefore, the remaining percentage must be taken, which can be calculated by subtraction:

• Water percentage = Total percentage - Percentage after heating.
• Water percentage = 100% - 78.6% = 21.4%

Answer 2

Question:

Options available;

a. 22%

b. 21.4%

c. 78.5%

d. 27%

Answer:

The percentage by mass of water in the crystal is 21.4%

The correct option is;

b. 21.4%

Step-by-step explanation:

From the question,

The initial mass of hydrate crystal = 6.235 g

Final mass of crystal = 4.90 g

Mass of water molecules displaced = 6.235 g - 4.90 g = g

Percentage by mass of water in the crystal is found from the following relationship;

1.335

Therefore;

`Percentage \ by \ mass = (1.335 \ g)/(6.235 \ g) * 100= 21.41 \% \approx 21.4\%`

The percentage by mass of water in the crystal = 21.4%.