QUESTIONS 1. Can you determine the molecular formula of a substance from its per

Question

QUESTIONS 1. Can you determine the molecular formula of a substance from its percent composition? 2. Given that zinc chloride has a formula weight of 136.28 amu, what is its formula? 3. Can you determine the atomic weights of zinc or copper by the methods used in this experiment? If so, how? What additional information is necessary to do this? 4. How many grams of zine chloride could be formed from the reaction of 3.57 g of zinc with excess HCT 5. Aluminum sulfide reacts with water to form aluminum hydroxide and hydrogen sulfide. (a) Write a balanced equation for this reaction. (b) How many grams of aluminum hydroxide can be obtained from 5.50 g of aluminum sulfide? Copyright e 2015 Pearson Education, Inc.

Explanation / Answer

1) yes, we can determine the molecular formula from percent composition by first calculating the empirical formula,

for example, C= 58.80%, H= 9.87 % and O=31.33 %,

Her ewe can divide the percent composition by mol. wt ,

we can get , c= 58.80/12= 4.9 ,

H= 9.87/1=9.87 ,

O= 31.33/16= 1.958 ,

NOW dividing all the values by lowest no.,

c=4.9/1.958= 2.50,

H= 9.84/1.958=5.04,

O= 1.958/1.958=1

NOW TO GET THE EVEN NO. OF CARBON WE CAN MULTIPLY IT BY 2,

c=2.50*2=5,H=5*2=10 AND O=1*2=2,

NOW THE MOLECULAR FORMULA BECOMES= C5H10O2,

2) ZINC CHLORIDE HAS THE FORMULA WT= 136.28 , ITS FORMULA IS AS,

ZnCl2,

where Zn = mass = 65.40,

Cl= 35.45*2=70.90,

(65.40+70.90= 136.3),

3) we can determine the Zn or Cu atomic mass by this method of calculating molecular formula from percent composition, here we need pecrentage composition to calculate the mass of zinc or Cu.

4) Zn= mol wt= 65.40 ,

From the 3.57 gms of Zn with reaction with excess HCl, we can calculate the zinc chloride formed as,

Zn + Hcl (Excess) --------> ZnCl2 +2H+,

here Zinc is the limiting reagent,

we can calculate the moles of zinc used as, 3.57/mol.wt,

= 3.57 gms/65.40 = 0.0545 moles ,

now we can calculate the therotical yield as,

=gms of reactant*mol wt of product/ mol wt of reactant,

= 3.57*136.3/65.40,

=486.591/65.40=7.44 gms of Zinc chloride (ZnCl2),

5) reaction of Aluminium sulfide with water,

first we write the formula of aluminium sulphide as,

Al2S3,

WE GET aluminium hydroxide Al(OH)3 ,

SO FOR GETTING Al(OH)3 , we will balance the reaction as,

Al2S3 + 6 H2O ---------> 2Al (OH)3 +3 H2S,

now we can calculate the gms of aluminium hydroxide obtained from 5.50 gms of Aluminium sulfide,

moles of Al2s3= 5.50/mol wt of Al2s3,

= 5.50/150.15=0.0366 moles,

here one mole of aluminium sulfide will give us two moles of aluminium hydroxide,

that is 0.0366 moles *2=0.0732 moles of aluminium hydroxide will be obtained,

= 0.0732*mol wt of aluminium hydroxide ,

=0.0732*78= 5.71 gms of Al(OH)3 will be obtained

6) copper (I) sulfide reacts with oxygen it gives copper sulfite and then it decomposes to Copper oxide and SO2 ,

THE BALANCED REACTIONS ARE ,

Cu2S + O2 ------> Cu2SO3 FOR THIS WE CAN BALANCE THE REACTION AS BELOW,

2Cu2S + 3 O2 -------> 2 CuSO3,

2 CuSO3 ---------> Cu2O + SO2, (decomposition of copper sulfite upon heating to get so2 and copper oxide),

Net reaction can be written as,

2 Cu2S +3 O2 ------> 2 Cu2O +2 SO2

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