# Calculation of water of crystallization

• lolbits
In summary, the conversation discusses an experiment where Na2SO4*10H2O was heated and some water evaporated, resulting in 1.8 grams of the substance. The question is then raised about the moles of water evaporated and the new formula for the substance. The calculations provided estimate that 0.011 mol of water evaporated and the new formula would be Na2SO4*8H2O. The accuracy of the calculations is noted to be dependent on the assumption that the substance was initially a decahydrate.
lolbits

## Homework Statement

We did an experiment at school. We heated $$Na_2SO_4*10H_2O$$ so that the water evaportated. We started with 2 gram of the substance, and ended up with 1.8 grams. Unfortunately, not all the water did evaporate because of the equipment we used at school. So the question is, a) how many moles of water evaporated and b) what would the new formula be?

mol = m(g) / M_m

## The Attempt at a Solution

a) Moles of water evaporated:
Know that 0.2 grams of water evaporated.
0.2/18 = 0.011 mol H2O evaporated.
That would be about 2 moles for each mol Na2SO4.

b) 2grams $$Na_2SO_4*10H_2O$$ $$\rightarrow$$ $$Na_2SO_4*xH_2O$$

$$M_m$$ of $$Na_2SO_4*10H_2O$$ is $$322g/mol$$

$$\frac{2g}{322g/mol}$$ gives $$0.0062 mol$$

$$M_m$$ of $$Na_2SO_4$$ = $$142 g/mol$$

$$\frac{1.8g}{0.0062 mol}=290.32 g/mol$$

$$290.32g/mol - 142g/mol = 148g/mol$$

$$M_m$$ of $$H_2O=18$$

$$\frac{142 g/mol}{18g/mol} = 8$$

So the new formula would be:

$$Na_2SO_4*8H_2O$$

Is this right?

No one? This should not be hard..

Calculations are almost OK, there is one weak point - you can't be sure you started with decahydrate, as amount of water may depend on many factors (including sample history - was it kept in open/closed bottle, what was humidity at the time and so on). But you can't check it, so your assumption is not bad.

The only thing I don't like is that you ignore accuracy - that is, some numbers are presented with guard digits, while other are presented already rounded down in a rather unpredictable way.

--

Borek said:
Calculations are almost OK, there is one weak point - you can't be sure you started with decahydrate, as amount of water may depend on many factors (including sample history - was it kept in open/closed bottle, what was humidity at the time and so on). But you can't check it, so your assumption is not bad.

The only thing I don't like is that you ignore accuracy - that is, some numbers are presented with guard digits, while other are presented already rounded down in a rather unpredictable way.

--

What I wrote was just quick. I will be more accurate next time. But again, thanks!

Yes, your solution is correct. It is important to note that the new formula for the hydrated compound would be Na2SO4*8H2O, as the original compound had 10 water molecules attached to it. This means that 2 water molecules evaporated during the experiment, and the remaining 8 water molecules are still attached to the compound. This new formula reflects the loss of 2 moles of water in the experiment. Great job!

## What is water of crystallization?

Water of crystallization, also known as water of hydration, is the water molecules that are chemically bonded to a crystal in a specific ratio. This water is essential for the crystal's stability and structure.

## Why is it important to calculate the water of crystallization?

Calculating the water of crystallization is important because it helps determine the purity and composition of a crystal. It also provides information about the crystal's structure and stability, which is crucial for various industrial and scientific applications.

## How is the water of crystallization calculated?

The water of crystallization can be calculated by finding the difference in weight of a crystal before and after heating it to remove the water molecules. The weight difference is then divided by the molecular weight of water (18 g/mol) to determine the number of water molecules per mole of the crystal.

## What factors can affect the calculation of water of crystallization?

The accuracy of the water of crystallization calculation can be impacted by various factors such as impurities in the crystal, incomplete removal of water molecules during heating, and variation in the crystal's structure or composition.

## What are some common methods used for the determination of water of crystallization?

The most commonly used methods for determining the water of crystallization include gravimetric analysis, Karl Fischer titration, and thermogravimetric analysis. Each method has its advantages and limitations, and the choice of method depends on the type of crystal and the level of accuracy required.

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