Lab report #4

Title: Identification of a compound by mass relations

Student’s name: Turdykhan A

Instructor’s name: Robert O’Relly

Lab report #4

Title: Identification of a compound by mass relationships

Aim:  To find out an unknown substance by mass relationships of species which are involved in chemical equation

Introduction:

Chemical equation is used to find the unknown compound, by relationship of masses of reactants and products that follows directly from the balanced equation for the reaction and the molar masses of species that are involved. In this experiment we must find the identity of unknown compound, which will be one of these four salts: NaHCO3, Na2CO3, KHCO3 , K2CO3. There are two experimental parts are used to distinguish between four compounds.

Firstly, the unknown compound will be heated. If the sample is carbonate, there will be no chemical reactions, therefore sample will lose not appreciable mass. If the sample is hydrogen carbonate, there will be a significant loss of mass, as some of the products will be escaped as gases. There are chemical equations of decomposition reactions of solid carbonate and solid hydrogen carbonate:

M2CO3 (s) M2CO3(s) (no change)

2 MHCO3(s) M2CO3 (s) + H2O(g) + CO2 (g) (mass is lost when gases escape)

Here, M is metal: sodium or potassium.

Secondly, the solid carbonate will be reacted with 6M hydrochloric acid to obtain chloride salt as main product. If the sample is solid carbonate, the equation between hydrochloric acid and solid carbonate will be:

M2CO3 (s) + 2 H+ (aq)+ 2Cl- (aq) 2 MCl (s) + H2O (g)+ CO2 (g) . (1)

If the sample is hydrogen carbonate, the equation between hydrochloric acid and solid hydrogen carbonate will be:

MHCO3 (s) + HCl (aq) MCl(s) + H2O (g) + CO2 (g) (2)

Here, M is metal: sodium or potassium.

To determine the unknown compound, the molar masses of reactants and products will be found. For each possible unknowns, there will be a different relationship between the mass of the original sample and chloride salt. Notice that, in the Reaction 1, in for every one mole of carbonate salt reacting, two moles of chloride salt are produced. In the case of Reaction 2, mole ratio between solid hydrogen carbonate and chloride salt is one to one. Then we need to calculate the change in mass that occurred between formed the chloride and hydrogen carbonate or carbonate. That difference divided by mass of the original salt will give us the “Q”. This value is different for each of the possible compounds. By comparison of experimental value of Q and theoretical values of Q of possible compounds, it will be determined identity of compound. After the first part of experiment, it will be determined whether the compound is carbonate or hydrogen

carbonate. Then you will be work of only two possible compounds which are will be the unknown compound.

Materials and methods:

1) Obtain one of the unknown compounds from the stockroom

2) Get a clean crucible with its cover and heat it gently with Bunsen burner flame for a minute or two, then strongly for two or more minutes. Allow the crucible to cool for about 10 minutes as, it must be cooled completely

3) Measure mass of the crucible and cover on analytical balance. Put about 0,5 g of unknown compound into crucible, then weigh mass of crucible and cover with unknown compound

4) Heat the crucible gently, gradually increasing the flame for ten minutes. After this, as crucible is cooled for about 10 minutes, the mass of crucible with its contents is measured

5) Put the crucible on the clay triangle, leaving the cover off. Add about 25 drops of 6M HCl to the sample, the next each drop should be added after subsiding the action of effervescence of sample. Continue this procedure till all of the solid will be dissolved.

6) Heat the sample gently for 10 minutes, placing the cover on the crucible in an off-center position. As the sample becomes dry, heat it increasing the flame intensity, then at a full flame strength for ten minutes. Cool it to room temperature, then weigh mass of crucible and cover with its contents.

At the end of experiments, dispose reaction products in appropriate places.

Results:

The number of unknown is 1.

Molar masses: NaHCO3 – 84,01 g, Na2CO3 – 105,99 g, NaCl- 58,44 g;

KHCO3 – 100,12 g, K2CO3 – 138,21 g, KCl- 74,55 g;

1) Distinguishing between carbonate and hydrogen carbonate.

Mass of crucible and cover plus unknown 84.5902 g

Mass of crucible and cover 83.9898 g

Mass of unknown before heating 84.5902 g – 83.9898 g = 0.6004 g

Mass of crucible, cover and unknown after heating 84. 3583 g

Loss of mass of sample 84,5902 g – 84.3583 g = 0.2319 g

Analysis.

If the compound is a carbonate, nothing much happens. At most, the sample will lose between 0.01 and 0.04 grams of mass. If you have a hydrogen carbonate, it will lose a significant amount of mass, usually greater than 0.1 grams. (http://www.austincc.edu/dsackett/CHEM1411/lab/1411exp5.pdf)

M2CO3(s) M2CO3(s) (no change)

2 MHCO3(s) M2CO3 (s) + H2O(g) + CO2 (g) (mass is lost when gases escape)

Here, M is metal: sodium or potassium.

Unknown is a hydrogen carbonate, as loss of mass of sample is greater than 0.1 g.

2) Distinguishing between sodium and potassium salts

Mass of crucible, cover and solid chloride 84.4076 g

Mass of solid chloride 0.4178 g

Change in mass( mass of chloride – original mass) 0.4178 g - 0.6004 g = 0.1826 g

Q = change in mass/ original mass

0,1826 g0.6004 g

= 0.3041

Analysis.

The compound is hydrogen carbonate, then the equation between hydrochloric acid and hydrogen carbonate would be:

MHCO3 (s) + HCl (aq) MCl(s) + H2O(g) + CO2 (g)

M is metal: sodium or potassium

1)The possible sodium compound would be NaHCO3.

One mole of compound (NaHCO3) one mole of NaCl

84.01 g of compound 58.44 g NaCl

Change in mass = 58.44 g - 84.01 g = 25.57 g. Q = 25.57 g / 84.01 g = 0.3044

2)The possible potassium compound would be KHCO3

One mole of compound (KHCO3) one mole of KCl

100.12 g of compound 58.44 g KCl

Change in mass = 58.44 g - 100.12 g = 41.68 g. Q = 41.68 g / 84.01 g = 0.4961

The unknown salt is NaHCO3. As experiment value of Q (0.3041) of compound is almost equal to theoretical value of Q (0.3044) of NaHCO3.

Percentage error = experimental value of Q−t h eoretical value of Q

t heoretical value of Q

Percentage error = (0.3041 – 0.3044) / 0.3044 = 0.0009855 X 100%= 0.0986 %

In case of KHCO3, percentage error = ( 0.3041- 0.4961) / 0.4961 = 0.3870 X 100% = 39.7 %

As 0.0986 % < 39.7 %, the unknown compound is sodium hydrogen carbonate.

Discussion.

The determined identity of compound is sodium hydrogen carbonate. The experimental value of Q of NaHCO3 was 0.3041, however the theoretical value of Q of NaHC03 was 0.3044. To find the identity of compound we have done two experimental procedures. Firstly, we heated the unknown solid to find whether this sample was carbonate or a hydrogen carbonate. Our sample was hydrogen carbonate, as loss of mass sample was 0.2319 g, due to loss of gases of CO2 and water. Secondly, to determine whether metal is sodium or potassium, we converted the solid hydrogen carbonate into chloride salt by reaction with hydrochloric acid. Then from our experimental data, we calculated the change in mass and Q of solid hydrogen carbonate. We had two possible compounds, namely sodium hydrogen carbonate and potassium hydrogen carbonate, in which one of them is the actual identity. The theoretical Q of sodium hydrogen carbonate was 0.3044 and theoretical Q of potassium hydrogen compound was 0.4961. Since the experimental value of Q of NaHC03 is almost equal to the theoretical value of NaHC03 (percentage error is 0.0986%), we determined that the unknown sample was NaHC03. There were some possible errors, due to them our obtained Q of sodium hydrogen carbonate(0.3041) is smaller than the theoretical value of Q of this compound(0.3044). One of the possible causes is that some mass may have been lost during HCl was added to sodium carbonate. When HCl was added to compound, the solution fizzled and some of sprayed out the crucible, therefore it affected to the mass of solid chloride (mass of solid chloride would be more than the obtained).

Conclusion

In this experiment, we have determined the unknown compound of 1 by mass relationships between the reactants and products, that it was the hydrogen carbonate NaHCO3, which was the actual identity.

Reference.

http://www.austincc.edu/dsackett/CHEM1411/lab/1411exp5.pdf