How many grams of NaCN would you need to dissolve in enough water to make exactly 375 mL of solution with a pH

Question

Sorry the pH is 10.00

Answer 1

I just had a question like this in a quiz, but I missed the significant figures! (I hate sig figs. xP)
Oh, and when I say something like "1.00E-4", that means
1.00 x 10^-4. Just in case you're not used to that notation.

First of all, you need the appropriate chemical equation:
NaCN + H2O --> HCN + OH-
Now, you can set up a chart
NaCN + H2O --> HCN + OH-
initial: ?M 0M 0M
change: -x +x +x
equilibrium: (?-x)M xM xM

We need to find the equilibrium concentration of NaCN.

We know that the pH is 10.00, so we can use this to find pOH.
pH + pOH = 14.00
pOH = 14.00 - 10.00
pOH = 4.00

Now that you know pOH, you can find the equilibrium concentration of OH-.
pOH= -log[OH-]
4.00 = -log[OH-]
-4.00 = log[OH-]
[OH-] = 1.00E-4

You can now fill this value in for the chart above.
NaCN + H2O --> HCN + OH-
initial: ?M 0M 0M
change: -1.00E-4M +1.00E-4M +1.00E-4
equilibrium: (?-1.00E-4)M 1.00E-4M 1.00E-4M

Remember, we needed to find the equilibrium concentration of NaCN. At equilibrium we see that the concentraion is the initial concentration of NaCN - 1.00E-4
The Kb value (you should be able to look this up in a chart in your book) is equal to 2.53E-5.

Kb = ( [HCN][OH-] ) / [NaCN]
We can now substitute in the equilibrium values we have found. This will allow us to find the unknown initial concentration of NaCN.

2.53E-5 = [1.00E-4][1.00E-4] / [x -1.00E-4]

Cross multiply to get:
(2.53E-5)x = 1.00E-8
x = 3.95E-4

So, the initial concentration of NaCN was 3.95E-4.

Remember that at equilibrium the concentraion of NaCN is the initial concentration of NaCN - 1.00E-4.

So, the equilibrium concentration of NaCN = 3.95E-4 + 1.00E-4
= 4.95E-4

Finally, we can find how many grams of NaCN we need. (The molar mass of NaCN is 49.0 g/mol, and 1 L = 1000 mL)
(4.95E-4 mol NaCN/ 1L) x (49.0 g NaCN/ 1mol NaCN) x(1 L/1000 mL) x (375 mL) = .00910 g NaCN

I hope this helped some! Sorry if it seems confusing, there's only so much I can show over the computer. =\

Answer 2

you need to find the atomic number i believe (i dont know the number at the bottom of the square in the periodic table) like hydrogen (H) is 1 and get Na,C,N and add them up then im not exactly sure what you do after that let me check in my notes i just got done doing those