I am struggling with organizing stoichiometry problems; I know how to solve them after they're set up. Here is an example:
"An organic compound with a FW of 417 was analyzed for ethoxyl {CH3-CH2O-} groups with the rxn:
ROCH2CH3 + HI --> ROH +CH3CH2I
CH3CH2I + Ag + H2O--> AgI +CH3CH2OH
A 25.42mg sample produced 29.03mg of AgI. How many ethoxl groups are there in each molecule?" Please help!
How many mols of the compound do you have? That is mols = g/molar mass = 0.02542 g/417 = ??
Now how many mols AgI was formed. That is
0.02903 g/molar mass AgI = zz
Therefore, you know zz mols AgI must have formed from zz mols CH3CH2I and in turn from (CH3CH2O)xR where x represents the number of the ethyoxyl groups in the original compound.
So what is x; that is, how many of the CH3CH2O- groups are in the original compound?
(mols ethoxyl groups/mols orig compound) = yy which will be close to a whole number. I worked the problem and came out with 2.02. Round that to a whole number, in this case, 2.00 and that will be the number of CH3CH2O- groups in the ROCH2CH3 compound at the beginning. Or to put it another way, the R(OCH2CH3)x compound must be R(OCH2CH3)2.
To solve stoichiometry problems like this, you need to follow a step-by-step approach. Here's how you can organize and solve this particular problem:
1. Determine the relevant balanced chemical equations:
- ROCH2CH3 + HI → ROH + CH3CH2I
- CH3CH2I + Ag + H2O → AgI + CH3CH2OH
2. Identify the given information:
- The molecular weight (FW) of the organic compound is 417.
- A 25.42 mg sample produced 29.03 mg of AgI.
3. Set up the conversion factors:
- Use the balanced equations to identify the molar ratios between different compounds.
- For example, in the first equation, the ratio between ROCH2CH3 and CH3CH2I is 1:1.
- In the second equation, the ratio between CH3CH2I and AgI is also 1:1.
4. Convert the given information into moles:
- Calculate the number of moles of AgI produced from the given mass (29.03 mg).
- To convert mg to g, divide by 1000: 29.03 mg /1000 = 0.02903 g.
- Use the molar mass of AgI (Ag = 107.87 g/mol, I = 126.90 g/mol) to convert grams of AgI to moles.
- Moles of AgI = (0.02903 g / 235.77 g/mol) = 0.000123 mol.
5. Use the stoichiometry ratios to find the number of moles of CH3CH2I:
- Since the ratio between CH3CH2I and AgI is 1:1, the moles of CH3CH2I will also be 0.000123 mol.
6. Calculate the number of moles of ROCH2CH3:
- Using the stoichiometry ratio from the first equation (1:1), the moles of ROCH2CH3 will also be 0.000123 mol.
7. Convert moles of ROCH2CH3 to grams:
- Multiply the moles of ROCH2CH3 by its molar mass to find the mass of ROCH2CH3, which is the same as the organic compound (417 g/mol).
- Mass of ROCH2CH3 = 0.000123 mol × 417 g/mol = 0.05139 g.
8. Finally, calculate the number of ethoxyl groups per molecule:
- Divide the mass of ROCH2CH3 by the molar mass of ROCH2CH3 to get the number of moles of ROCH2CH3.
- Divide the moles of ROCH2CH3 obtained above by the number of molecules in one mole of ROCH2CH3 to get the number of ethoxyl groups per molecule.
By following these steps, you should be able to find the answer to the question.
To solve this stoichiometry problem, let's break it down step-by-step:
Step 1: Determine the molar masses
First, we need to determine the molar masses of the relevant compounds involved in the reaction:
- Molar mass of ROCH2CH3: This is not explicitly given in the question, so we'll need to calculate it. From the structure provided, we can determine the molar mass as follows:
1 atom of Carbon (C) weighs 12.01 g/mol
3 atoms of Hydrogen (H) weigh 3.01 g/mol (3 * 1.01 g/mol)
1 atom of Oxygen (O) weighs 16.00 g/mol
We add up the masses to get: 12.01 g/mol + 3.01 g/mol + 16.00 g/mol = 31.02 g/mol
- Molar mass of AgI: Ag (silver) has a molar mass of 107.87 g/mol, and I (iodine) has a molar mass of 126.90 g/mol. Adding these together, we get: 107.87 g/mol + 126.90 g/mol = 234.77 g/mol.
- Molar mass of CH3CH2OH: C (carbon) has a molar mass of 12.01 g/mol, H (hydrogen) has a molar mass of 1.01 g/mol, and O (oxygen) has a molar mass of 16.00 g/mol. Adding these together, we get: 12.01 g/mol + (2 * 1.01 g/mol) + 16.00 g/mol = 46.07 g/mol.
Step 2: Calculate the number of moles
Next, we need to calculate the number of moles for the relevant compounds involved in the reaction:
- Moles of AgI: Divide the mass of AgI given in the question (29.03 mg) by its molar mass (234.77 g/mol):
(29.03 mg / 1000 mg/g) / 234.77 g/mol = 0.0001237 mol
Step 3: Determine the stoichiometry
Now, let's examine the balanced chemical equation to determine the stoichiometric ratios between the compounds involved:
2 C2H5OH + 2 HI → 2 C2H5I + 2 H2O
2 C2H5I + Ag + H2O → 2 C2H5OH + AgI
From the balanced equation, we see that 2 moles of AgI are produced from 2 moles of C2H5I. Therefore, there is a 1:1 stoichiometric ratio between AgI and C2H5I.
Step 4: Calculate the moles of C2H5I
Since there is a 1:1 stoichiometric ratio between AgI and C2H5I, the moles of C2H5I are also 0.0001237 mol.
Step 5: Determine the moles of ROCH2CH3
Given this information, we can use the balanced equation to determine the stoichiometric ratio between C2H5I and ROCH2CH3. From the balanced equation, we see that 2 moles of C2H5I are produced from 1 mole of ROCH2CH3.
Therefore, the moles of ROCH2CH3 are:
0.0001237 mol C2H5I * (1 mol ROCH2CH3 / 2 mol C2H5I) = 0.00006185 mol ROCH2CH3
Step 6: Calculate the number of moles of the organic compound
Since the molar mass of ROCH2CH3 is 31.02 g/mol, we can use this value to calculate the number of moles of the organic compound:
0.00006185 mol ROCH2CH3 * 31.02 g/mol = 0.001918 g
Step 7: Determine the number of ethoxyl groups
Finally, we need to determine how many ethoxyl {CH3-CH2O-} groups are present in each molecule of the organic compound.
Given that the sample contains 25.42 mg, we can calculate the number of ethoxyl {CH3-CH2O-} groups using the following equation:
(25.42 mg / 1000 mg/g) / 0.001918 g/mol = 13.26 ethoxyl groups
Therefore, there are approximately 13.26 ethoxyl groups in each molecule of the organic compound.
I hope this step-by-step guide helps you with organizing stoichiometry problems. If you have any further questions, feel free to ask.