[isotope124]

Hi everyone,I just wanted to ask if you can help me with some of the problems that I have for homework.Although they are simple to most people, to me and some of my classmates , they are not, primarily because of our teacher.Thank you in advance.

1.How many moles of Calcium chloride hexahydrate is in 1,5 g of that salt?

2.How many molecules of gas does 1 dm3 contain in environment of 20ºC and 100 kPa?

3.Chemical compound was discovered and it consists of sulfur and oxygen.The percentage of sulfur is 40%.Find the simplest formula of that compound.


Please note that these problems were translated from my language so there might be some things that I didn`t translate well.Feel free to ask anything.

[svncehm]

Hi there.

1. calculate the molar mass of CaCl2*6H20 and use formula for moles then n=m/M.
2. Use basic gas equation (p*V=n*R*T). R is a constant 8,31 kPa*L/mol*K.
3. Now I really don't remember the procedure for that type of problem.

But keep it on.
 

[Borek]

Please remember you have to show your attempts to receive help, this is a forum policy.

[KungKemi]

Hi everyone,I just wanted to ask if you can help me with some of the problems that I have for homework.Although they are simple to most people, to me and some of my classmates , they are not, primarily because of our teacher.Thank you in advance.

1.How many moles of Calcium chloride hexahydrate is in 1,5 g of that salt?

2.How many molecules of gas does 1 dm3 contain in environment of 20ºC and 100 kPa?

3.Chemical compound was discovered and it consists of sulfur and oxygen.The percentage of sulfur is 40%.Find the simplest formula of that compound.


Please note that these problems were translated from my language so there might be some things that I didn`t translate well.Feel free to ask anything.

I can't give you the answer to these exact questions, but I can show you techniques necessary to solve similar questions to the following:

Q1: How many moles of sulphuric acid (H₂SO₄) are there in a 2.50-g sample?

Okay, so let's list our knowns:

m_H2SO4 = 2.50 g
molar mass H₂SO₄ = (2×1.01 + 4×16.00 + 1×32.06) = 98.08 gmol^-1
moles H₂SO₄ = ? mol

Okay, so the periodic table lists all the molar masses of all the elements (for example Hydrogen is 1.01 g). Now, these elemental masses are expressed in terms of 1 mole of a given element, so... 1 mole of Hydrogen has a total mass of 1.01 g in this case.

As for this question, 1 mole of sulphuric acid has a mass of 98.08 g. Naturally, if our sample had a mass of 98.08 g it would be expected that there would be 1 mole of sulphuric acid present, however, this is not the case. As such, we use the following conversion:

m_H2SO4 × 1 mol H₂SO₄/98.08 g H₂SO₄ = 2.50 g × 1 mol/98.08 g = 0.025 mol H₂SO₄...

So, 2.50-g of sulphuric acid is equivalent to 0.025 moles of sulphuric acid.

Q2: If gas is placed in a 1.5 dm³ rigid container, at 25°C and a pressure of 123 kPa, how many moles of gas are present?

Okay, so like before, let's list the knowns:

Volume gas = 1.5 dm³ = 1.5 L
Temperature = 25°C = 298 K
Pressure = 123 kPa
moles gas = ? mol

Now, in order to find moles of gas we will need to use a special equation known as the ideal gas equation: PV = nRT. In the following, we know P (123 kPa), we know T (298 K), we know V (1.5 L), and we know R (the ideal gas constant - 8.31). Now, what we do not know (and what we are trying to find) is n (moles of gas). As such, rearrange to find n...

n = PV/RT = (123 × 1.5)/(8.31 × 298) = 0.075 mol gas

So, we found that 0.075 moles of gas must be present in order for the following conditions (P, V, T) to be met. Now, in your question there is one extra step - convert moles to molecules. You can do this through use of Avogadro's constant.

Q3: A compound, after elemental analysis, was found to consist of 85.6% C, and 14.4% H. Determine its empirical formula.

Now, in the following questions the percentage composition (more often then not) is expressed in terms of a mass percentage. As such, we can express the percentage parts in terms of mass, so...let

85.6% C = 85.6 g C, and 14.4% H = 14.4 g H...

Remember that a chemical formula is always expressed in terms of moles, so an empirical formula is the lowest whole-mole ratio of a chemical formula. As such, convert mass to moles using a conversion similar to Q1...

85.6 g C × 1 mol C/12.0 g C = 7.13 moles C
14.4 g H × 1 mol H/1.0 g H = 14.4 moles H

We don't like decimals so divide the two by the lowest number of moles (7.13)...

7.13 moles C/7.13 = 1.0 mol C
14.4 moles H/7.13 = 2.0 mol H...

Therefore, the empirical formula of the compound is C₁H₂, or CH₂.

I hope that this helps, and I hope that I didn't break the Forum rules.
KungKemi

[Vidya]

Hi everyone,I just wanted to ask if you can help me with some of the problems that I have for

problems posted by you are very simple mathematical problems.You just need to know some relations
like in one mole of any substance is its molar .Then you should know how to calculate molar mass.If you are good in math then it becomes very simple.
so try it because it is an important starting problem in learning about the mole concept and you can not go higher in chemistry if you can not do it.
so go from 2.50 g H2SO4 to moles using ratio that molar mass of H2SO4 in g is one mole of H2SO4
Do many problems based on it to practice this concept.