Chemistry Problem III

1. Sep 26, 2007

higherme

Question:

Given
mass of hexaammine complex [Ni(NH3)6](NO3)2 is 0.0937 g
mass of Ni2+ is 0.0193 g

if the mass of [Ni(NH3)6](NO3)2 was 0.0886 g instead of 0.937g, this would indicate that the hexaammine complex was not pure and some ammonia had been lost. Calculate the value of n in the formula [Ni(NH3)n](NO3)2

my attempt:

- Take the mass of Ni2+ and find the moles of it using Ni2+ molar mass. This mole would then be the same as the moles of the complex (1:1 ratio) --> i found to be 0.0003288 moles

- then take the grams of the complex 0.0886 g and divide by that mole: 0.0886g/0.0003288moles = 264.4404 g/mol which would be the molar mass of the complex

- then i subtracted the molar masses of Ni2+ and (NO3)2 from that molar mass and what is left is the amount of (NH3)n. Then i took that amount and divided by the molar mass of one NH3 to see how many NH3 I have.

am I thinking right?

2. Sep 27, 2007

chemisttree

You are well on your way.

3. Sep 29, 2007

higherme

i keep getting the wrong answer....what am i doing wrong?

4. Sep 29, 2007

higherme

so here is what i did

0.0193g Ni x (1 mol Ni/58.693g) x (1mol [Ni(Nh3)6](NO3)2/ 1 mol Ni) = 3.288 E-4 moles of [Ni(Nh3)6](NO3)2

then i took 0.0886g and divide by this moles to get the molar mass
0.0886g/3.288 E-4 moles [Ni(Nh3)n](NO3)2
= 269.44 g/mol

then i substracted Ni (58.693g/mol) and (NO3)2 (124.008g/mol) from that molar mass
269.44 - 58.693 - 124.008
86.739 g/mol

then take this number and divide by (NH3) molar to see how many NH3 there are:
(86.739g/mol) / (17.031g/mol)
= 5.09 NH3

so n is equal to 5.09.... this answer is still incorrect when i entered into the computer for grading ><

what am I doing wrong!!!!!!!!!??????????

5. Sep 29, 2007

eli64

did you enter 5 or 5.09? n should be a whole number

6. Oct 2, 2007

chemisttree

How many significant figures are you asked to provide? Generally you would use an integer value if you were trying to determine the empirical formula for a pure substance. In this case the sample may be a mixture of the hexamine, pentamine, etc... and so a non-integer value for n may be correct.