## A superconductor is a substance that is able to conduct electricity without resistance

Question: A superconductor is a substance that is able to conduct electricity without resistance, a property that is very desirable in the construction of large electromagnets. Metals have this property if cooled to temperatures a few degrees above absolute zero, but this requires the use of expensive liquid helium (boiling point 4 K). Scientists have discovered materials that become superconductors at higher temperatures, but they are ceramics. Their brittle nature has so far prevented them from being made into long wires. A recently discovered compound of magnesium and boron, which consists of 52.9% Mg and 47.1% B, shows special promise as a high-temperature superconductor because it is inexpensive to make and can be fabricated into wire relatively easily. What is the formula of this compound?

Solution:

Let us take 100 g of compound
Then, mass of Mg = 52.9g
and mass of boron = 47.1g
Moles of Mg = Mass/Molar Mass = 52.9g/24.31g/mol
= 2.18 mol
and moles of B = 47.1g/10.81g/mol
= 4.36 mol
So, the molar ratio of Mg and B:
nMg/nB = 2.18 mol/4.36 mol = 1/2
Formula of the compound is MgB2

## A student attempts to identify an unknown compound by the method used in this experiment. She finds that when she heated a sample weighing 0.4862 g

1. A student attempts to identify an unknown compound by the method used in this experiment. She finds that when she heated a sample weighing 0.4862 g the mass barely changed, dropping to 0.4855 g. When the product was converted to a chloride, the mass went up, to 0.5247 g.

a. Is the sample a carbonate? NO

b. What are the two compounds that might be in the unknown? KHCO3 and NaHCO3

c. Write the chemical equation for the overall reaction that would occur when the original compound was converted to a chloride. If the compound is a hydrogen carbonate, use the sum of Reactions 1 and 2. If the sample is a carbonate, use Reaction 2. Write the equation for a sodium salt and then for a potassium salt.

d. How many moles of the chloride salt would be produced from one mole of original compound? _____

e. How many grams of the chloride salt would be produced from one molar mass of original compound? Molar masses: NaHCO3 84.008 g Na2CO3 105.99 g NaCl 58.44 g KHCO3 100.118 g K2CO3 138.21 g KCl 74.55 g

If a sodium salt, ____________ g original compound → ____________ g chloride I

f a potassium salt, ____________ g original compound → ____________ g chloride

f. What is the theoretical value of Q, if she has an Na salt? ____________ if she has a K salt? ____________

g. What was the observed value of Q? ____________

h. Which compound did she have as an unknown?

I have bolded my own answers. I need help with the rest, please show work!

Solution:

a) As decomposition temperature is very high ( barely change its weight at high temperature) then the compound is expected as carbonate .

So the first answer is “YES”

b) So, the probable compound is Na2CO3 or K2CO3

g)observed value is 0.5247g, close to expected value with K​​​​​​2​​​CO3​​ ,0.5345g.

h) So the compound was K​​​​​​2​​​​​CO​​​3  which also correlates with high decomposition temperature of this compound (1200°C).