Problem 435.
How many milliliters of concentrated of hydrochloric acid(p \u003d 1.19 g / ml), containing 38% (mass.) HCI, must be taken to prepare 1 l of 2n. solution?
Solution:
M (HCI) \u003d M E (HCI) \u003d 36.5 g / mol.
Calculate the mass of HCl in 1 liter of 2n of its solution: 2 . 36.5 \u003d 72.93g.
Calculate the mass of a 38% solution using the formula:

where

The volume of solution that must be taken to prepare 1 liter of a 2n solution is calculated by the formula:

m (p-pa) = p . V,

where p

Answer: 161.28 ml.

Problem 436.
To 100 ml of 96% (by weight) H 2 SO 4 (density 1.84 g/ml) was added 400 ml of water. A solution with a density of 1.220 g/ml was obtained. Calculate its equivalent concentration and mass fraction of H 2 SO 4 .
Solution:
The mass of a solution of 100 ml of a 96% solution is found by the formula:

m(p-pa) = p . V,

where p is the density, and V is the volume of the solution, we get:

m (p-pa) = 1.84 . 100 = 184

The mass of sulfuric acid in this solution is found by the formula:

where
- mass fraction of the dissolved substance; m (in-va) - the mass of the dissolved substance; m (r-ra) - the mass of the solution.

Calculate the mass of the solution obtained by mixing 100 ml of a 96% solution with 400 ml of water, we get:

m" (p-pa) = (100 + 400) . 1.220 = 610 g

Let us determine the molar mass of the equivalent of H2SO)4 from the ratio:

M E (B) - molar mass of the acid equivalent, g / mol; M(B) is the molar mass of the acid; Z(B) - equivalent number; Z (acids) is equal to the number of H + ions, H 2 SO 4 (((((2.

Then we find the equivalent concentration of the solution by the formula:

where
m (B) is the mass of the dissolved substance, M E (V) is the molar mass of the equivalent of the dissolved substance, V is the volume of the solution (in l or ml).

Calculate the mass fraction of the resulting solution:

Answer: 7.2n; 28.96%.

m(p-pa) = p . V,

where p is the density, and V is the volume of the solution, we get:

m (p-pa) = 1.18 . 1000 = 1180

Calculate the mass of hydrochloric acid in solution using the formula:

where
- mass fraction of the dissolved substance; m (in-va) - the mass of the dissolved substance; m (r-ra) - the mass of the solution.

Let us determine the molar mass of the equivalent of HCl from the relation:

M E (B) - molar mass of the acid equivalent, g / mol; M(B) is the molar mass of the acid; Z(B) - equivalent number; Z (acids) is equal to the number of H + ions, H 2 SO 4 → 2.

Answer: 11.8n.

Problem 438.
What is the volume of 10% (by mass) sulfuric acid ( p\u003d 1.07 g / ml) will be required to neutralize a solution containing 16.0 g of NaOH?
Solution:
The reaction equation for the neutralization of a NaOH solution with a solution of H 2 SO 4 has the form:

H 2 SO 4 + 2NaOH ↔ Na 2 SO4 + 2H 2 O

It follows from the reaction equation that 0.5 moles of NaOH are spent to neutralize 1 mole of NaOH, which means that the equivalent mass of sulfuric acid in this reaction is 49 g / mol (M / 2 \u003d 98/2 \u003d 49).

Now we calculate the mass of sulfuric acid required to neutralize 16 g of NaOH from the proportion:

The mass of a solution containing 19.6 g of H 2 SO 4 is calculated by the formula:

where
- mass fraction of the dissolved substance; m (in-va) - the mass of the dissolved substance; m (r-ra) - the mass of the solution.

The volume of the solution is calculated by the formula:

m (p-pa) = p . V,

where is the density, and V is the volume of the solution, we get:

Answer: 183.18 ml.

The mass fraction of the element ω (E)% is the ratio of the mass of a given element m (E) in a taken molecule of a substance to the molecular weight of this substance Mr (in-va).

The mass fraction of an element is expressed in fractions of a unit or as a percentage:

ω (E) \u003d m (E) / Mr (in-va) (1)

ω% (E) \u003d m (E) 100% / Mr (in-va)

The sum of mass fractions of all elements of a substance is equal to 1 or 100%.

As a rule, to calculate the mass fraction of an element, a portion of a substance is taken equal to the molar mass of the substance, then the mass of a given element in this portion is equal to its molar mass multiplied by the number of atoms of a given element in a molecule.

So, for a substance A x B y in fractions of a unit:

ω (A) \u003d Ar (E) X / Mr (in-va) (2)

From proportion (2) we derive the calculation formula for determining the indices (x, y) in chemical formula substances, if the mass fractions of both elements and the molar mass of the substance are known:

X \u003d ω% (A) Mr (in-va) / Ar (E) 100% (3)

Dividing ω% (A) by ω% (B), i.e. transforming formula (2), we obtain:

ω(A) / ω(B) = X Ar(A) / Y Ar(B) (4)

The calculation formula (4) can be transformed as follows:

X: Y \u003d ω% (A) / Ar (A) : ω% (B) / Ar (B) \u003d X (A) : Y (B) (5)

Calculation formulas (3) and (5) are used to determine the formula of the substance.

If the number of atoms in a molecule of a substance for one of the elements and its mass fraction are known, the molar mass of the substance can be determined:

Mr(in-va) \u003d Ar (E) X / W (A)

Examples of solving problems for calculating the mass fractions of chemical elements in a complex substance

Calculation of mass fractions of chemical elements in a complex substance

Example 1. Determine the mass fractions of chemical elements in sulfuric acid H 2 SO 4 and express them as a percentage.

Solution

1. Calculate the relative molecular weight sulfuric acid:

Mr (H 2 SO 4) \u003d 1 2 + 32 + 16 4 \u003d 98

2. We calculate the mass fractions of elements.

To do this, the numerical value of the mass of the element (taking into account the index) is divided by the molar mass of the substance:

Taking this into account and denoting the mass fraction of the element with the letter ω, the calculations of mass fractions are carried out as follows:

ω(H) = 2: 98 = 0.0204, or 2.04%;

ω(S) = 32: 98 = 0.3265, or 32.65%;

ω(O) \u003d 64: 98 \u003d 0.6531, or 65.31%

Example 2. Determine the mass fractions of chemical elements in aluminum oxide Al 2 O 3 and express them as a percentage.

Solution

1. Calculate the relative molecular weight of aluminum oxide:

Mr(Al 2 O 3) \u003d 27 2 + 16 3 \u003d 102

2. We calculate the mass fractions of elements:

ω(Al) = 54: 102 = 0.53 = 53%

ω(O) = 48: 102 = 0.47 = 47%

How to calculate the mass fraction of a substance in a crystalline hydrate

The mass fraction of a substance is the ratio of the mass of a given substance in the system to the mass of the entire system, i.e. ω(X) = m(X) / m,

where ω(X) - mass fraction of substance X,

m(X) - mass of substance X,

m - mass of the whole system

Mass fraction is a dimensionless quantity. It is expressed as a fraction of a unit or as a percentage.

Example 1. Determine the mass fraction of water of crystallization in barium chloride dihydrate BaCl 2 2H 2 O.

Solution

Molar mass BaCl 2 2H 2 O is:

M (BaCl 2 2H 2 O) \u003d 137 + 2 35.5 + 2 18 \u003d 244 g / mol

From the formula BaCl 2 2H 2 O it follows that 1 mol of barium chloride dihydrate contains 2 mol H 2 O. From this, we can determine the mass of water contained in BaCl 2 2H 2 O:

m(H2O) = 2 18 = 36 g.

We find the mass fraction of water of crystallization in barium chloride dihydrate BaCl 2 2H 2 O.

ω (H 2 O) \u003d m (H 2 O) / m (BaCl 2 2H 2 O) \u003d 36 / 244 \u003d 0.1475 \u003d 14.75%.

Example 2. From sample rock weighing 25 g, containing the mineral argentite Ag 2 S, silver is isolated weighing 5.4 g. Determine the mass fraction of argentite in the sample.

A) Magnesium burning cool Ice melting C) River sand settling in water
D) Mixing sulfur and iron powders E) Boiling water

2. The molar mass of iron is
A) 26 g/mol cool 56 g/mol C) 52 g/mol D) 112 g/mol E) 56

3. In the formula 2Na2S, the number of sodium and sulfur atoms are equal
A) 1 and 2 cool 4 and 1 C) 2 and 4 D) 4 and 2 E) 2 and 1

4. Formula of Mn(VII) oxide
1. MnO2 cool Mn2O7 C) Mn2O3 D) MnO3 E) MnO

5. In the reaction scheme P+O2 ? P2O5 need to put the coefficients
A) 4, 5, 2 cool 2, 1, 1 C) 2, 5, 2 D 5, 4, 2 E) 2, 4, 5

6. The substitution reaction equation is -
A) 4Na + O2 = 2 Na2O cool CaCO3 = CaO +CO2? C) Zn + CuS = ZnS + Cu
D) 2Mg + O2 = 2MgO E) 2H2 + O2 > 2H2O

7. An iron nail immersed in a solution of copper chloride (II) is covered with a red coating of copper. This is an example of a reaction:
A) Exchange cool Decomposition C) Substitution D) Connection E) No such reaction

8. Symbol chemical element manganese
A) ?e cool Mg C) O D) Mn E) Mr

9. About the chemical element, and not about the simple substance nitrogen in question in expression
A) Nitrogen is integral part air cool composition nitric acid HNO3 enters nitrogen
C) N2 nitrogen formula D) Liquid nitrogen is sometimes used to freeze food
E) Nitrogen inert gas
10. Aluminum does not have a physical property
A) Electrical conductivity cool Thermal conductivity C) Silver white color
D) Ability to be magnetized E) Gas under normal conditions

11. A sign that allows you to name the rusting of a nail chemical reaction- this is:
A) Heat evolution cool Gas evolution C) Discoloration
D) Odor E) Precipitation

12. Iron sulfide is a complex substance, not a mixture because
A) It can be separated by a magnet into iron and sulfur
cool It can be separated by distillation into iron and sulfur
C) It consists of atoms of a different chemical element and cannot be separated by physical methods into iron and sulfur
D) It is insoluble in water E) a gas under normal conditions

13. 3.01 * 10 23 iron atoms make up
A) 2 mol cool 3 mol C) 1 mol D) 0.5 mol E) 1.5 mol

14. 69 g of sodium is
A) 3 mol cool 1 mol C) 6.3 mol D) 1.5 mol E) 0.5 mol

15. Filtering can separate the mixture:
A) copper and iron chips cool sugar and water C) chalk and water
D) water and acetic acid E) water and gasoline

16. The interaction of magnesium with oxygen refers to the reactions:
A) cool exchange decomposition C) compound D) substitution E) no such reaction

17. K chemical phenomena relate:
A) marble grinding cool water evaporation C) ice melting D) copper melting E) coal combustion

19. What is the valency of aluminum?
A) 1 cool 2 C) 3 D) 4 E) 5

20. Units of measurement of molar mass:
A) grams cool gram/mol C) mol D) melogram E) no unit of measurement

21. The molar mass of NaHCO3 is:
A) 156 cool 156 g/mol C) 84 g/mol D) 84 E) 84 L

22. Indicate the decomposition reaction:
A) 2H2 + O2 > 2 H2O cool 2Na + 2H2O > 2NaOH + H2
C) C + O2 > CO2 D) 2NH3 > N2 + 3H2
E) AgNO3 + HCl > AgCl + HNO3

23. The mass fraction of oxygen in sulfuric acid H2SO4 is approximately:
A) 16% cool 33% C) 65% D) 2% E) 17%

25. In which of these rows are only metals located?
A) K, Zn, Fe cool Si, Ca, Bi C)Al, C, Cr D) W, Os, B E) P, Au, Pb

26. The mass fraction of sulfur in SO2 is:
A) 32% cool 64% C) 50% D) 80% E) 12%

27. The mass of zinc sulfide formed by heating 10 g of sulfur with zinc is:
A) 12 g cool 30.31 g C) 25.6 g D) 10.5 g E) 32.4 g

28. Symbol for the chemical element krypton
A) Ca cool Kr C) K D) Cd E) C

29. Substance is
A) Air B) copper C) Mirror D) Granite E) milk

30. The list of physical properties is superfluous
A) Cool burning density C) Thermal conductivity
D) Boiling point E) Melting point