# Mass calculation and Theoretical yield help?

Calculate the mass of potassium dichromate (K2Cr2O7) required to produce 1.0 g of iodine (I2) according to the equation below:

K2Cr2O7 + 6NaI + 7H2SO4 à Cr2 (SO4)3 + 3I2 + 7H2O + 3Na2SO4 + K2SO4

two--Calculate the theoretical yield percentage of a reaction between sodium and water, if 25.0 g of sodium react with excess water and if 15.5 L of gaseous hydrogen are collected at NTP conditions

Relevance

K2Cr2O7 + 6NaI + 7H2SO4 --> Cr2(SO4)3 + 3I2 + 7H2O + 3Na2SO4 + K2SO4

Part1: You want to produce 1g of I2. The molecular mass of I2 is 2*126.90447g/mol = 253.80894/mol

1g I2 * (1mol / 253.80894g) = 0.00393997154mol I2

Now, using the balanced chemical equation, you can convert moles of I2 to moles of K2Cr2O7

0.00393997154mol I2 * (1mol K2Cr2O7 / 3mol I2) = 0.00131332385mol K2Cr2O7

Now, convert the moles of K2Cr2O7 back to mass, using its molecular mass 294.1846 g/mol.

0.00131332385mol K2Cr2O7 * (294.1846 g/mol) = 0.386359651g K2Cr2O4.

Part2: I'm going to assume the chemical reaction is Na + H2O --> NaOH + H2. Balancing this gives

2Na + 2H2O --> 2NaOH + H2

Since you have excess water, sodium is the limiting reagent. Its molecular mass is 22.99g/mol.

25g Na * (1mol / 22.99g) = 1.08742932mol Na.

Converting this to moles of Hydrogen gas using the equation, we get

1.08742932mol Na * (1mol H2 / 2 mol Na) = 0.54371466mol H2.

This is the theoretical yield.

Keeping this in mind for later, let's get the actual yield. Use the following equation, using the constant R=0.0821 and keeping in mind that NTP (Normal Temperature and Pressure) is equal to 293K and 1atm, respectively:

PV=nRT

(1)(15.5) = n(0.0821)(293)

n = 15.5 / (0.0821 * 293) = 0.644348647mol H2

So, theoretical percentage yield refers to actual yield over theoretical yield.

If we crunch the numbers, TPY = (0.644348647/0.54371466) * 100% = 118.508603% yield.

Source(s): It is strange that the percent yield is over 100%. I could have made an error, but I did my best to check my work.
• because the actually yield is received by an attempt (as adverse to calculated from the periodic table like the theoretical yield) you likely measured some thing incorrect, or used too a lot or a substance or your stability would were off. it truly is likely why your actually yield became off.