chemistry question

Question

hello i really need help filling this out

To prepare for a lab session, a lab assistant needs to make a 1 liter solution of each of the substances listed below.  Fill in the missing parts of the table for each substance by first calculating the formula (molar) mass for each compound, and then use the number of moles given to calculate the mass the assistant should mass out OR use the mass given to solve for the number of moles this would be.  For each calculation, work must be shown to earn credit. Be sure to include the formula mass of water for CaCl2 • 2H2O and Na(CH3)2 AsO2 • 3H2O. 

Substance	Formula (molar) mass	(g/mol)	Number of Moles	Mass (g)
NaCl		0.14
KCl		0.0050
C3H6O3			1.01
Na(CH3)2 AsO2•3H2O		0.0262
CaCl2•2H2O			0.15
KH2PO4			1.4

NaCl 

 

 

KCl 

 

 

C3H6O3 

 

 

Na(CH3)2 AsO2•3H2O 

 

 

CaCl2•2H2O 

 

 

KH2PO4 

Answer 1

Hello Brian!

(It seems like there may have been a formatting issue with the table as posted, specifically it seems to have separated the units for the molar masses into a separate column by mistake. I am going to assume that the numerical data given should be shifted one column to the right, as that makes the most sense. If this is incorrect, please let me know what the original table looked like and I will fix my response as soon as I can!)

In general, the molar mass (or formula mass) of a compound is determined by adding up the atomic masses of each atom in the compound's chemical formula. Atomic masses for each element can be found from the periodic table.

For example, the molar mass of potassium dihydrogen phosphate (KH₂PO₄) would be:

MW = 1*M_K + 2*M_H + 1*M_P + 4*M_O

MW = 1*(39.10 g/mol) + 2*(1.008 g/mol) + 1*(30.97 g/mol) + 4*(16.00 g/mol)

MW = 136.086 g/mol

In the case of CaCl₂•2H₂O and Na(CH₃)₂AsO₂•3H₂O, make sure to include the water molecules as part of the chemical formula for each compound. Also remember that for anything in parentheses, like (CH₃)₂ in the latter compound, the outer subscript applies to the entire group. So (CH₃)₂ represents 2 carbon atoms and 6 hydrogen atoms total (two groups, each containing 1 carbon and 3 hydrogen). Similarly, the coefficient in front of water shows that there are three molecules of water present in each formula unit of the compound, each of which contains 2 hydrogen atoms and 1 oxygen atom.

Thus, the molar mass of Na(CH₃)₂AsO₂•3H₂O would be:

MW = 1*M_Na + 2*M_C + 12*M_H + 1*M_As + 5*M_O

MW = 1*(22.99 g/mol) + 2*(12.01 g/mol) + 12*(1.008 g/mol) + 1*(74.92 g/mol) + 5*(16.00 g/mol)

MW = 214.026 g/mol

Now that we have the molar mass for each compound, we can use it to convert between mass (in grams) and number (in moles). Using m to denote mass, n to denote moles, and MW to denote molar mass for a compound, we can relate these three values using the following equation:

m = n*MW

Which is useful when we know how many moles of the compound we have and need to find what mass that corresponds to. This should make sense based on the units of molar mass, g/mol. If a compound has a molar mass of 50 g/mol, 5 mol of that compound should have a mass of 250 g, since each mole of the compound has a mass of 50 g and there are 5 moles total. Next, a little algebra shows us that:

n = m/MW

Which is useful when we have the mass of our compound and need to find how many moles of the compound that mass equates to. Returning to the two examples above:

n_KH2PO4 = m_KH2PO4 / MW_KH2PO4

n_KH2PO4 = (1.4 g) / (136.086 g/mol)

n_KH2PO4 = 0.0102876... mol

m_{Na(CH3)2AsO2•3H2O} = n_{Na(CH3)2AsO2•3H2O} * MW_{Na(CH3)2AsO2•3H2O}

m_{Na(CH3)2AsO2•3H2O} = (0.0262 g) * (214.026 g/mol)

m_{Na(CH3)2AsO2•3H2O} = 5.60748... g

Remember to round the final answers according to the rules for significant figures! The periodic table I used was accurate to the 100ths place, so all of my molar masses will be accurate to the 100ths place as well (since molar mass is based on addition). The number of moles of KH₂PO₄ is given to 2 significant figures, so the mass of KH₂PO₄ should be reported to 2 significant figures as well (since we multiplied inexact numbers). The mass of Na(CH₃)₂AsO₂•3H₂O is given to 3 significant figures, so the number of moles of Na(CH₃)₂AsO₂•3H₂O should be reported to 3 significant figures as well.

Thus:

MW_KH2PO4 = 136.07 g/mol

n_KH2PO4 = 0.010 mol

m_KH2PO4 = 1.4 g (Given)

MW_{Na(CH3)2AsO2•3H2O} = 214.03 g/mol

n_{Na(CH3)2AsO2•3H2O} = 0.0262 mol (Given)

m_{Na(CH3)2AsO2•3H2O} = 5.61 g

The rest of the table works exactly the same way. If anything still seems unclear or if you have any questions, let me know and I will try to clarify.

I hope this helps!

Tim W.