If 20.0 mL of a 0.0800 M HNO3, 35.0 mL of a 0.0800 M KSCN, and 40.0 mL of a 0.0800 M Fe(NO3)3 are combined, what is the initial concentration of SCN- in the mixture

0.0295M

Explanation:

As you can see, in the mixture you have KSCN and other compounds. The KSCN in solution is dissolved in K⁺ ions and SCN⁻ ions. That means initial concentration of SCN⁻ ions is the same of KSCN, 0.0800M.

You are adding 35.0mL of this solution and the total volume of the mixture is 20.0mL + 35.0mL + 40.0mL = 95.0mL.

That means you are diluting your solution 95.0mL / 35.0mL = 2.714 times.

And the concentration of SCN⁻ is:

0.0800M / 2.714 =

"you start with 15.67 g mgco3-xh2o, when heated and all the water is removed, you are left with 7.58 g mgco3 only.

therefore, you had 15.67 - 7.58 = 8.09g h2o  

moles of each :

7.58g mgco3 = ? moles = 7.58g / 83.4g/mole = 0.09moles  

8 09g h2o = ? moles = 8.09g / 18g/mole = 0.44moles  

molar ratio between the two by dividing both numbers by the smallest number :

0.09 / 0.09 = 1  

0.44 / 0.09 = 4.8  

mgco3-5h2o"