How do you identify the unknown substance as a percentage hydrate lab?

Answer 1

The formula for percent hydrate is calculated by taking the mass of the water lost and dividing by the mass of the hydrate.

1. To calculate the water lost, subtract the value for the mass of the anhydrate (substance after it has been heated and dried [without water]) from the value for the mass of the hydrate (substance before it has been heated [with water]).

Mass hydrate - mass anhydrate = water lost

OR, another way to put it

Mass before heating - mass after heating = water lost

2. Next, divide the mass of the water lost by the mass of the hydrate.

Mass water lost

- mass hydrate

Ex. A student was asked to identify an unknown hydrate by following the procedure described in this module. After heating and cooling, a 2.752g sample of this unknown weighed 1.941g. Students were given a list of possible compounds from which to identify their unknowns:

LiNO3 * 3 H2O

Ca(NO3)2 * 4 H2O

Sr(NO3)2 * 4 H2O

In order to identify the unknown substance, the percent hydrate for the following compounds listed above needs to be calculated. This is done by first calculating the molar mass of each hydrate, then calculating the percent of water in each hydrate.

Before proceeding, the amount of water lost needs to be calculated, followed by calculating the percent water in the unknown hydrate.

Water lost: 2.752g hydrate - 1.941g anhydrate = 0.811g

Percent water of hydrate: 29.47%

0.811g

2.751g x 100%

1. Identify the molar mass of each hydrate.LiNO3 * 3 H2O (Lithium Nitrate Trihydrate)

Molar Mass of LiNO3 (Lithium Nitrate): 68.9459g

Molar Mass of 3 H2O (Water): 18.0152 * 3 OR 54.0456g

Molar Mass of LiNO3 * 3 H2O = 122.9915g (add values above, not multiply!)

Ca(NO3)2 * 4 H2O (Calcium Nitrate Tetrahydrate)

Molar Mass of Ca(NO3)2 (Calcium Nitrate): 164.0878g

Molar Mass of 4 H2O (Water): 18.0152 * 4 OR 72.0608g

Molar Mass of Ca(NO3)2 * 4 H2O = 236.1486g

Sr(NO3)2 * 4 H2O (Strontium Nitrate Tetrahydrate)

Molar Mass of Sr(NO3)2 (Calcium Nitrate): 211.6298g

Molar Mass of 4 H2O (Water): 18.0152 * 4 OR 72.0608g

Molar Mass of Sr(NO3)2 * 4 H2O = 283.6906g

2. Calculate the percent water in each hydrate.Percent Water of Hydrate LiNO3 * 3 H2O (Lithium Nitrate Trihydrate)

- 54.0456g

122.9915g x 100 = 43.94%

Percent Water of Hydrate Ca(NO3)2 * 4 H2O (Calcium Nitrate Tetrahydrate)

- 72.0608g

236.1486g x 100 = 30.52%

Percent Water of Hydrate Sr(NO3)2 * 4 H2O (Strontium Nitrate Tetrahydrate)

- 72.0608g

283.6906g x 100 = 25.40%

Due to the closest comparison in percentage values, the most likely compound is Ca(NO3)2 * 4 H2O (Calcium Nitrate Tetrahydrate). The values aren't exact matches probably because the water was not fully evaporated while being heated (leading calculations to indicate less water than there really was/is in substance). In other words, due to errors made on behalf of the experiment conductor by not completely heating the substance.

Calculating Molar Mass of Substances

Element A: Number Atoms Element A x Atomic Mass Element A = Mass A

Element B: Number Atoms Element B x Atomic Mass Element B = Mass B

etc.

Add all of the mass values found above (Mass A + Mass B ...) and you have the value of molar mass. In the case above, to find the molar mass of the hydrates, simply add the molar mass of water (multiply by however many water molecules are in compound) and the molar mass of the other compound.

Answer 2

1. Calculate the amount of water lost.

Mass Hydrate - Mass Anhydrate

hydrate = with water (before heating)

anhydrate = without water (after heating)

2. Calculate the percent of water.

Divide the amount of water lost by the mass of the hydrate; multiply by 100%.

Mass water lost

- Mass hydrate x 100%

Answer 3

1. Find the amount of water lost.

Subtract the mass of the anhydrate (substance after it's been heated [meaning no water left]) from the mass of the hydrate (substance before it's been heated [meaning water is present]).

Water lost = mass hydrate - mass anhydrate (mass before - mass after).

2. Find the percent water lost in hydrate.

Divide the amount of water lost by the mass of the hydrate; multiply result by 100%.

- water lost

mass hydrate x 100%

Answer 4

percent water= (mass of water lost/ mass of hydrate) x 100

Answer 5

By weighing the sample before and after heating to dehydration.