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% Composition and Empirical Formula Lab

Zixuan Huang


Percent composition shows the amount of each substance in the entire sample. This allows the scientist to analyze and identify unknown substances.


Determine the % composition by mass and the empirical formula of the hydrated compound of the hydrate MgSO2 and H2O.



● Bunsen burner

● crucible and cover

● crucible tongs

● MgSO4 and H2O

● distilled water

● dropper or micropipet

● ring and pipe-stem triangle

● ring stand

● spatula

● stirring rod, glass

● weighing paper





Magnesium sulfate (MgSO4)

White crystalline,

Dissolves in water solid, odorless,

density of 2.66


melting point of 1,124°C

The dust of this product has a

stimulating effect on mucous


Dihydrogen Monoxide


transparent liquid,odorless, melting point is 0.00°C,

boiling point is 100.00 °C

Heat capacity and heats of

vaporization and fusion

Non-irritating to the eyes. Skin: Non-irritating to the skin. Ingestion: No hazard

expected in normal industrial use.


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In order to know the change in the mass of the hydrate in the crucible after the weight of the crucible is removed in the follow-up, we need to measure the weight of the empty crucible with a balance in gram and in order to more easily observe the change in the mass of the substance, we need to make it accurate to 2 decimal.Which it’s weight will be 17.38g.


Put 1.640g of hydrate into the cup with a spoon and reweight the crucible.Which would get 19.00g.


Place a triangle on the iron ring and move the ring clamp 1 inch away from the bunsen burner, then turn it on.


Place the crucible on the clay triangle by using the tong and allow the flame to heat it.Make sure all the crucible gets burned, not just a single part.If the hydrate starts to smoke, turn the fire smaller.


At 2mins,6mis,3mis of heating, remove the crucible and cool it.


Put the crucible on the table. And feel whether it’s cold above the top of the crucible with your bare hands. Once the crucible is cool, weigh the crucible again.


After the third time we heated, the mass of the crucible is stable at 18.16g which means evaporation is complete.

Observation and Data collection

Observation:By heating the crucible, the hydrate inside will begin to melt, changing from the original translucent solid to a white solid. After repeated heating, the Water in the hydrate will evaporate, and the crucible mass will drop, starting at 19.00 and ending at 18.16g.

Heat Time Flame distance Mass of the crucible 1st 2.000mins 1 inch above top of flame 18.46g

2nd 6.000mins 1 inch above top of flam 18.16g

3rd 3.000mins 1 inch above top of flame 18.16g

Data Analysis Determine the composition of hydrated compounds of the hydrate MgSO2 andnH2O, should use this formula:

Unknown value:

1)Total mass of the Hydrate(MgSO4·nH2O):1.620g

2)Total mass of nH2O:0.840g

3)Total mass of MgSO4:0.780g

Use the third trial to calculate the empirical formula:


0.780 grams/ 120.365 mol/grams= 0.0067mol


0.840 grams/ 18.02 mol/grams=0.0466 mol

Result of n:

MgSO4: 0.0067mol/0.0067=1 mol

H2O: 0.0466mol/0.0067mol=6.955mol


Use the empirical formula to calculate the composition of MgSO4 and H2O in hydrate.And we should use this formula:

Molar mass of MgSO4·7H2O



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The main purpose of this "Perform% Composition and Empirical formula of hydrate lab" is to find out its empirical formula on the basis of only knowing the weight of the substance, and then we can even more deeply find the composition of each compound in the substance.This experiment mainly adopts the means of material transformation when heated. Hydrate is the general term for MgSO4·nH2O. After the water in the hydrate is burned to a certain amount of heat, it will evaporate into a gas and volatilize into the air. We can use the weight of evaporated water to infer the mass of the hydrate, the mass of MgSO4, and of course the mass of H2O. These are the data we need to use to calculate the formula.

When preparing Lab materials, we need to list all the things that can be used to facilitate the people behind to follow our experimental procedures. Then we have to draw a diagram of how the main tools are used to make it easier to understand. The

most important thing is that these tools must be kept clean and dry, otherwise slight impurities will cause deviations in subsequent experiments

In the part of the experimental steps to obtain this data, I summarized some of the experiences that made the data more accurate and the mistakes we made. When heating, ensure that the entire crucible is evenly heated, otherwise it will cause some of the water in the hydrate to evaporate, but some will not. This is a point that has a great influence on the experimental results. It should be noted that each measurement must wait for the scale to return to zero. The most important point is also the one that is most easily overlooked. When measuring heated substances, you need to wait for the crucible to cool down before measuring. This is also to ensure the accuracy of the data. The crucible has not cooled down yet, and the water in it is still volatilizing. This is not stable data. So this step is an important detail. This is also the point we made a mistake. We didn't let the crucible completely cool down, so we were anxious to weigh it. After the hydrate is heated many times and the water inside is completely evaporated, the weight of the substance will stabilize at a constant value. At this time, only MgSO4 is left in the crucible, and then the empty crucible is subtracted from this gram weight to get the weight of water. The experiment time is within our expected range, and we estimate that it will take about ten minutes. Finally, we did get the weight of the substance after the H2O was completely evaporated within ten minutes, that is, after the second heating. But in order to continue to confirm whether all the water has evaporated, we performed a third heating. When we finally got the same data, we breathed a sigh of relief. What I want to say is that in the experiment, at least two pieces of data are needed to get two equal weights to ensure that all the water evaporates.

After collecting the experimental data, how to organize them is also a very important thing. We need to list all the conditions for obtaining the data. For example, the heating time, the number of times. The weight of the object and the distance from the fire. The different values of these things will affect the accuracy of the final calculation of the formula. We need to think from the perspective of others, what conditions and results they need to get to make this experiment logical and can be rigorously studied and applied.

After everything is ready, we have the data we need, we can start to bring the data into the formula, calculate the% mass percentage composition and the empirical formula of the hydrated MgSO2 and H2O hydrated compounds. First, we need to know the empirical formula, and then use this formula to calculate the% combination. Because the data we have now is the weight of certain compound, but

% composition of an Element = (Total mass of Element in Compound)/(Total mass of the Compound) x 100. So we need to calculate how many moles each compound has to know the total mass of the compound.Back to the first step how to calculate the empirical formula. The principle we need to apply is to divide the existing compound weight by the molar mass of the compound, but the molecular formula is currently obtained, so in the end we need to put it at the minimum 1 moles per compound. If the mole ratio is still not available If it is close to an integer, we also need to multiply it to an integer. After that, we can import the data into the% of composition formula.When calculating, we need to pay more attention to the significant figure, the decimal point of each value must be the same from beginning to end, and the more significant figures, the finer the final calculated result. I suggest that the sinification figure should be as fine as 4 digits at least because I found that if it is only as fine as 3 digits, then I got a very biased empirical formula.

Error Analysis


Error 1:At weighting the mass, we only get the value accurate to 4 significant digits, so the calculation of the empirical formula is not so accurate.

Propagation:Because we are only accurate to four significant figures, small numerical changes due to rounding during calculation will cause very large changes in the final result. So in order to avoid this kind of thing from happening, we'd better make the value more accurate to more than five significant figures

Error 2:When weighing, we did not completely cool down the cup, the measured data may be a little bit biased, which will lead to a bias in the final data when calculating.

Propagation:Because we haven't completely cooled the cup, the water that should have been evaporated during this heating has not evaporated completely, which will cause the Mass of MgSO4 and H2O we get to be inaccurate, and finally lead to deviations in the calculation. In order to avoid this situation, we recommend that the cup be placed on the table to cool for five minutes, and finally take the palm of the hand and feel it cool before taking the measurement.

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