Skip to main content

Chemistry exercise: mass needed to prepare a solution

One of the most easy exercises in chemistry is calculating concentrations of solutions. However, for a lot of starting students or people new in the field, it remains a hassle to carry out such exercise.

Let's say we want to prepare a 0,250 M aqueous solution of calcium chloride (CaCl2). CaCl2 is a white powder at room temperature. It dissolves readily in water.

The exercise is as follows:

What mass of CaCl2 is required to prepare a 300,0 mL of a 0,250 M solution? (Molecular mass of CaCl2 = 110,98 g/mol)

Approach this step by step. First, deduct what is expected.

1. We need to calculate the amount of mass needed. Mass is in gram (g). The solution has a concentration of 0,250 M. This means we want a concentration of 0,250 Molar or = mol/liter.

2. 0,250 mol/liter. But we do not want 1 liter, we want 300 milliliter, or 0,3 liter.
Calculate the amount of mol we need: 0,250 mol/liter to 300 milliter, I calculate it as follows:

0,250 mol divide by 1000 (1 liter = 1000 milliliter). We have 0,00025/1 milliliter.
We need 300 milliliter, so multiply by 300.

0,00025 * 300 = 0,075 mol/300 milliter.

3. Now we can calculate the mass needed of CaCl2. We use the following equation to calculate the mass:

n = m/M

with n = mol; m = mass; M = molecular mass

Fill in the equation, and we get:

0,075 mol = m/110,98 g/mol
m = 0,075 mol * 110,98 g/mol
m = 8,3235 g

4. That's it! We need to weigh 8,3235 g of CaCl2 and dissolve it in 300 mL of water to obtain a 0,250 M solution.

5. Overview:

- Calculate amount of mol needed in the solution.
- Calculate the mass needed by using the mol equation.
- Dissolve the mass in the given amount.



 

Comments

Popular posts from this blog

Alkylhalides: Substitution reactions 6 (Sn1)

Alkylhalides: Substitution Nucleophile substitution reaction ( Sn1 reaction ) Sn1 side reactions I will now discuss some side reactions that can occur when a Sn1 reaction takes place. Carbocation shift Illustrated in the scheme below: Benzyl- and allylhalides  Benzyl- and allylhalides can undergo Sn1 AND Sn2 reactions. How to distinct them? Sn1 conditions: protic solvent and by adding a weak attacking nucleophile. Note: Benzyl- and allylhalides easily undergo Sn1 reactions, because their carbocations are very stable. Sn2 conditions: aprotic solvent and by adding a strong attacking nucleophile. Note: tertiary benzylhalides and tertiary allylhalides will NOT undergo a Sn2 reaction because of the steric effects (see chapter Sn2 reaction blogposts). Sn2 reaction examples Sn1 reaction examples Sn1 and Sn2 reactions in biology, nature and medicines S-Adenosyl methionine This is a biological methylating agens, also known as SAM . It is a frequen...

Alkynes: addition of H2 gas and Lindlar catalyst

Alkynes: addition of Hydrogen gas (H2) Performing a catalytic reduction on an alkyn will result in giving an alkane. The alkene intermediate will be formed in the process, but will immediately react into an alkane. The end result is just the formed alkane, without stacking of the alkene intermediate. The Lindlar catalyst In theory this is a "poisoned or defected" catalyst. If you use a normal catalyst you will get the above effect with your alkyne. The Lindlar catalyst contains Palladium combined with Calcium-carbonate and treated with Lead. Palladium is the actual catalyst, the calcium carbonate is the carrier of the substance and the lead is the poisonous compound. Using the Lindlar catalyst instead of another catalyst together with Hydrogen gas, will lead into forming the cis-alkene intermediate instead of the forming of the corresponding alkane. 

Erythropoietin: definition, structure, synthesis in vivo

Definition: EPO: = erythropoietin   A glycoprotein hormon-like structure, a sialoglycoprotein, which is an important factor in the survival, growth and proliferation of erythroid precursor cells (EPC) and it improves the de novo creation, differentation and growth of red blood cells (RBC). Thus, EPO controls the erythropoiesis = production of RBC. EPC: these are cells that are located in the bone marrow, will eventually form the RBC. RBC: cells responsible for the transport and distribution of oxygen throughout the body. Structure: Built out of 165 aminoacids (AA). They are all connected and form 1 polypeptide chain.  Although, within the chain, there are 2 disulfide bonds.  Respectively on positions: Cys7-161 and Cys29-33  Cys = cystein and the numbers indicate the positions these AA are located. Cystein structure. Available  sulfide  group for bonding There are also 4 positions where there is a possibility for glycosylation  Namel...