We start with an unknown sample and are told to make a 100mL solution that is 10

Question

We start with an unknown sample and are told to make a 100mL solution that is 100ppm Ca2+ by assuming 50% weight of CaCO3.

So we weighed out .0510g of the unknown and dissolved it in 100mL of water. We then took 10mL of that solution and diluted to 100mL . Then we took 20mL of that solution and diluted to 100mL again. Using Flame atomic absorption spectroscopy we calculated the concentration to be 4.197146ppm Ca. I need to find the percent mass of Calcium in the unknown sample using this data but I can't figure out how. Also the percent should be around 20-40% based on previous labs using the same unknown. I keep coming up with really bad nunbers like 4 percent and I have no idea what I am doing wrong if someone could point me in the right direction?

Explanation / Answer

start back calculation.....

let x= 4.197146 ppm = 4.197146 *10^-6 g in 1g of soln= 4.197146 *10^-4 g in 100 g soln ........

here amount of sample is very less ,hence density of solution could be assumed to density of water= 1g/ml.........

therefore. x g is present in 100ml final sample= 100g ..........

therefore in x g is present in the 20 g of previous sample.... so in the 100 g of previous sample 5x g would be present..........

i.e. 5x g is present in the 2nd sample i.e 10ml samlple......so in 100ml of it 5*10x= 50x g would be present,

i.e. in 0.0510 g of unknown 5x g of Ca is present = 50*4.197146 *10^-4 g in 0.0510 g = 41.1%

Related Questions

Navigate

• Browse (All)
• Browse W
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.