[Nolan]

I have a lab where I determined the empirical formula of Magnesium Chloride to be Mg₂Cl₇ (really off, I know). How would I go about calculating the percent error? I read somewhere that I should use the molar masses but then I end up getting a 211% error.

Thanks in advance.

[chenbeier]

How do you calculate it show your work.

[Nolan]

How do you calculate it show your work.

Mass Cl in compound: 1.84g – 0.30g = 1.54g

Cl: 1.54g / 35.45gmol-1 = 0.0434mol    0.0434mol / 0.0123mol = 3.53    3.53 * 2 = 7.06 ≈ 7
Mg: 0.30g / 24.31gmol-1 = 0.0123mol    0.0123mol / 0.0123mol = 1    1 * 2 = 2

Experimental Empirical Formula: Mg2Cl7

Accepted Empirical Formula: MgCl2

Molar Mass of MgCl2: 24.31gmol-1 + (35.45gmol-1 * 2) = 95.21gmol-1

Molar Mass of Mg2Cl7:  (24.31gmol-1 * 2) + (35.45gmol-1 * 7) = 296.77gmol-1

% Error: [|(95.21gmol-1 – 296.77gmol-1)| / 95.21gmol-1] * 100 = 211.7%

[chenbeier]

Where does 1,84 g and 0,3 g comes from.

[AWK]

Did you check the water content?

[Nolan]

Where does 1,84 g and 0,3 g comes from.

measured values in the experiment. We combined Mg with HCl (diluted) and then dehydrated the sample. We measured 0.3g of Mg at the start and 1.84g of Mg_xCl_x at the end.

[Nolan]

Did you check the water content?

No, but my teacher told me not to worry about that.

[chenbeier]

The question is was it dehydrated enough.

The theoretical calculation would be 1.17 g. 1.84 g is to heavy.

[Nolan]

The question is was it dehydrated enough.

The theoretical calculation would be 1.17 g. 1.84 g is to heavy.

Right, but I would mention that in the discussion of the report. All I want to know is how to calculate the percent error.

[AWK]

You got MgCl_3.5 instead of MgCl₂

[Nolan]

You got MgCl_3.5 instead of MgCl₂

So my error would be around 105% instead of 211%, right?

Thank you very much.

[AWK]

For me, 1.5/2 gives 75 % error

[Nolan]

For me, 1.5/2 gives 75 % error

How did you calculate your error? That's what I'm looking for because I'm doing error by molar mass but I don't know if that's wrong or not.

[AWK]

Calculate the Percent Error in an Experimental Empirical Formula

Your expected empirical formula is MgCl₂ - two chlorine atoms. You got 3.5. Error is 1.5.

[Nolan]

Thank you so much! I know that I have a lot of questions but thanks for putting up with me!